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Prerequisites: we assume that the reader is familiar with the CDC log table and Basic operations in CDC documents.
As explained in CDC log table, each atomic non-primary-key column of the base table gets two corresponding columns in the log table:
one with the same name and type; we call this the value column,
one with the original name prefixed with cdc$deleted_ and of boolean type; we call this the deletion column.
Atomic means any column whose type is not a non-frozen collection, non-frozen UDT, or a counter. Examples of atomic column types are: int, timeuuid, text, frozen<map<int, int>>, frozen<my_type> (where my_type was created using a CREATE TYPE statement). Examples of non-atomic types are: map<int, int>, set<int>, my_type.
These columns are filled in CDC log delta rows created for UPDATE, INSERT, and column DELETE statements, as explained in the basic operations document. If the statement writes a non-null value to the column, its corresponding value column in the delta row gets filled with that exact value. If the statement writes a null, its corresponding deletion column in the delta row gets filled with True.
For example:
CREATE TABLE ks.t (pk int, ck int, v int, PRIMARY KEY (pk, ck)) WITH cdc = {'enabled': true};
UPDATE ks.t SET v = 0 WHERE pk = 0 AND ck = 0;
UPDATE ks.t SET v = null WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v" from ks.t_scylla_cdc_log;
gives:
cdc$time | pk | ck | v | cdc$deleted_v
--------------------------------------+----+----+------+---------------
9d764f58-2f32-11eb-ac58-84f095630d16 | 0 | 0 | 0 | null
9d768b58-2f32-11eb-5cf2-f8131733ca0b | 0 | 0 | null | True
The situation is more complicated for non-atomic columns. Each different case has its own section below.
Note: everything in this section applies only to non-frozen maps. Frozen maps are atomic and behave in the same way as other atomic types (such as int). The same comment applies to sets and lists in sections below.
Let’s start with an example.
CREATE TABLE ks.t (pk int, ck int, v map<int, text>, PRIMARY KEY (pk, ck)) WITH cdc = {'enabled': true};
DESCRIBE TABLE ks.t_scylla_cdc_log;
gives:
CREATE TABLE ks.t_scylla_cdc_log (
...
"cdc$deleted_elements_v" frozen<set<int>>,
"cdc$deleted_v" boolean,
...
v frozen<map<int, text>>,
...
We replaced the irrelevant lines with .... The important information for us is the set of columns created for the v map<int, text> column; there are three:
v: it has the same name as the corresponding base column and its type is a frozen version of the base column’s type; the base type was map<int, text>, so this column’s type is frozen<map<int, text>>,
cdc$deleted_v: its name is the base column’s name prefixed with cdc$deleted_, and its type is boolean,
cdc$deleted_elements_v: its name is the base column’s name prefixed with cdc$deleted_elements_, and its type is a frozen set whose element type is the base column’s key type. Since the base column’s type was map<int, text>, meaning that its key type is int, we obtained frozen<set<int>>.
The general rule is: for each column X map<K, V> in the base table, where X is a column name and K, V are types, the CDC log table has 3 corresponding columns:
X frozen<map<K, V>>, which we call the added elements column,
cdc$deleted_X boolean, which we call the collection deletion column,
cdc$deleted_elements_X frozen<set<K>>, which we call the deleted elements column.
The added elements column is filled when new elements are added to the map. Continuing our example:
UPDATE ks.t SET v = v + {1: 'v1', 2: 'v2'} WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+--------------------+---------------+------------------------
e3ef7952-2f3f-11eb-dc8e-210f916d0a5b | 0 | 0 | {1: 'v1', 2: 'v2'} | null | null
The deleted elements column is filled when elements are removed from the map:
UPDATE ks.t SET v = v - {1, 2, 3} WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+------+---------------+------------------------
5bb26094-2f40-11eb-63e9-a0d4519f9c1b | 0 | 0 | null | null | {1, 2, 3}
Note that the elements don’t need to exist to be removed. Removing an element is expressed by adding a special tombstone value (as usual in ScyllaDB) under the given key. Thus, we can understand the cdc$deleted_elements_X column as showing the set of keys which were assigned tombstones in the corresponding statement. Recall that a tombstone removes a value if its timestamp is greater than or equal to the value’s timestamp.
Deleting values for specific keys in CQL as above can only be done using an UPDATE statement with the - {...} notation.
The collection deletion column is set to True when a collection-wide tombstone is created; the tombstone removes all elements with timestamps lower than or equal to the tombstone’s timestamp. To delete an entire collection, either set the entire collection value to null, or to {}; both methods have the same effect:
UPDATE ks.t SET v = null WHERE pk = 0 AND ck = 0;
UPDATE ks.t SET v = {} WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+------+---------------+------------------------
2fcc5e48-2f41-11eb-eb46-b59d059991a9 | 0 | 0 | null | True | null
2fcca81c-2f41-11eb-2332-1c30f8a2f717 | 0 | 0 | null | True | null
A single UPDATE statement can be used to overwrite a collection using the = {...} notation. Overwriting is the same as creating a collection-wide tombstone and adding new elements at the same time. In other words, the following statement:
UPDATE ks.t SET v = {1: 'v1', 2: 'v2'} WHERE pk = 0 AND ck = 0;
is equivalent to the following:
BEGIN UNLOGGED BATCH;
UPDATE ks.t SET v = {} WHERE pk = 0 AND ck = 0;
UPDATE ks.t SET v = v + {1: 'v1', 2: 'v2'} WHERE pk = 0 AND ck = 0;
APPLY BATCH;
CDC understands this equivalence. Overwriting a map column X is expressed in CDC by setting both the X and cdc$deleted_X columns. For example:
BEGIN UNLOGGED BATCH
UPDATE ks.t SET v = {} WHERE pk = 0 AND ck = 0;
UPDATE ks.t SET v = v + {1: 'v1', 2: 'v2'} WHERE pk = 0 AND ck = 0;
APPLY BATCH;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+--------------------+---------------+------------------------
555a4034-2f42-11eb-3e76-a6bbe7fa31b2 | 0 | 0 | {1: 'v1', 2: 'v2'} | True | null
The same result would be obtained if we used a single SET v = {1: 'v1', 2: 'v2'} statement.
Deleting a map can be understood as a special case of overwriting with 0 elements.
INSERT statements can only overwrite (or delete) maps; with INSERT, there is no way to add elements to a map without overwriting it, since there is no notation for INSERT that would correspond to the UPDATE-specific SET X = X + {...} notation. There is also no way to remove specific keys with an INSERT.
Example of using an INSERT, compared with an UPDATE:
INSERT INTO ks.t (pk, ck, v) VALUES (0, 0, {1: 'v1', 2: 'v2'});
UPDATE ks.t SET v = {1: 'v1', 2: 'v2'} WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v", "cdc$operation" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v | cdc$operation
--------------------------------------+----+----+--------------------+---------------+------------------------+---------------
90dd349e-2f43-11eb-fce8-46a663386323 | 0 | 0 | {1: 'v1', 2: 'v2'} | True | null | 2
90dd91b4-2f43-11eb-7ee7-e37ab84122d4 | 0 | 0 | {1: 'v1', 2: 'v2'} | True | null | 1
As we can see, the effect on the v-related columns for INSERT is the same as for UPDATE. Which statement was used can be determined by the cdc$operation column. The exact difference between the two types of statements was explained in the basic operations document.
The examples in this section use maps, but the discussion applies to all non-frozen collections: maps, sets, and lists, and even to non-frozen UDTs.
Executing the following statements:
BEGIN UNLOGGED BATCH
UPDATE ks.t SET v = v + {1: 'v1', 2: 'v2'} WHERE pk = 0 AND ck = 0;
UPDATE ks.t SET v = {} WHERE pk = 0 AND ck = 0;
APPLY BATCH;
SELECT * from ks.t;
is equivalent to:
UPDATE ks.t SET v = {1: 'v1', 2: 'v2'} WHERE pk = 0 AND ck = 0;
SELECT * from ks.t;
and gives:
pk | ck | v
----+----+--------------------
0 | 0 | {1: 'v1', 2: 'v2'}
We’ve explained that the SET v = {...} notation creates a collection-wide tombstone, and tombstones delete all values that have timestamps lower than or equal to the tombstone’s timestamp. How is it then possible to both delete a collection and add elements to it in the same statement?
Each CQL statement that arrives to ScyllaDB comes with a timestamp (or multiple timestamps, in case of specially constructed batches, but that’s rare); generally, it is the timestamp that’s assigned to the written data. However, collection-wide tombstones written by UPDATE ... SET X = {...} statements or INSERT statements are an exception.
The rule is as follows:
Given an
UPDATEor anINSERTstatement, if the timestamp of this statement isT, then the timestamp of collection-wide tombstones written by this statement isT - 1.
This is what makes the above behavior possible. Suppose that the statement
UPDATE ks.t SET v = {1: 'v1', 2: 'v2'} WHERE pk = 0 AND ck = 0;
has timestamp T. It is translated by ScyllaDB into 3 pieces of information: an element (1, 'v1') with timestamp T, an element (2, 'v2') with timestamp T, and a collection-wide tombstone with timestamp T-1. The tombstone will therefore remove all elements that have timestamps lower than or equal to T - 1, but will not remove the elements (1, 'v1'), (2, 'v2'), since their timestamps are greater.
Warning: this rule does not apply when deleting collections using a column DELETE. In that case, the original timestamp is used. The following example illustrates that:
BEGIN UNLOGGED BATCH
DELETE v FROM ks.t WHERE pk = 0 AND ck = 0;
UPDATE ks.t SET v = v + {1: 'v1', 2: 'v2'} WHERE pk = 0 AND ck = 0;
APPLY BATCH;
SELECT * from ks.t;
gives:
pk | ck | v
----+----+---
(0 rows)
In this example, the elements (1, 'v1'), (2, 'v2') were given the same timestamp as the tombstone coming from the DELETE statement (by default, all statements in a BATCH are given the same timestamp), hence they were deleted by that tombstone.
CDC takes this rule into account when calculating the value of the cdc$time column. Recall from the CDC log table document that the cdc$time column is a timeuuid that contains a timestamp. Usually, when we write a cell to the base table, a corresponding row in the CDC log table appears whose cdc$time is a timeuuid whose timestamp is equal to the timestamp of the written cell. However, collection-wide tombstones are an exception; because of the above rule, the CDC log row which informs about the tombstone (i.e. one which has cdc$deleted_X column set to True) has cdc$time such that its timestamp is the timestamp of the collection-wide tombstone increased by 1.
We already saw this phenomenon before. Consider the following example, where we pick the timestamp manually:
UPDATE ks.t USING TIMESTAMP 1606390225588947 SET v = {1: 'v1', 2: 'v2'} WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v
--------------------------------------+----+----+--------------------+---------------
c72c7c3e-2fda-11eb-ba1d-e2d8e9bb0299 | 0 | 0 | {1: 'v1', 2: 'v2'} | True
There is a single row that informs both about the collection-wide tombstone and the elements (1, 'v1'), (2, 'v2') written by the UPDATE, even though the tombstone and the elements have different timestamps, as previously explained. The timestamp of this timeuuid in microseconds (which we can obtain using the Python snippet from the aforementioned document) is 1606390225588947, i.e. the one we picked. By our previous discussion, this means that the timestamp of the elements is 1606390225588947 and the timestamp of the collection-wide tombstone is 1606390225588946.
This is how CDC compensates for the collection-wide tombstone rule. If it didn’t, i.e. if it instead always enforced that cdc$time’s timestamp is the same as the timestamp of all elements and tombstones of the base write, the above example would have to produce two rows with different cdc$time values, one for the collection-wide tombstone and one for the elements.
This compensation may introduce confusion when using column deletes, which are an exception to the rule. When we delete a collection column using a DELETE statement, the original timestamp of the write is used, but CDC still applies the compensation, i.e. the cdc$time’s timestamp is then equal to the timestamp of the DELETE increased by 1:
DELETE v FROM ks.t USING TIMESTAMP 1606390225588947 WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v
--------------------------------------+----+----+------+---------------
c72c7c48-2fda-11eb-1510-6f4271ad8200 | 0 | 0 | null | True
the timestamp of this timeuuid is 1606390225588948, i.e. it is the timestamp used in the delete statement increased by 1.
Another example:
BEGIN UNLOGGED BATCH
DELETE v FROM ks.t USING TIMESTAMP 1606390225588946 WHERE pk = 0 AND ck = 0;
UPDATE ks.t USING TIMESTAMP 1606390225588947 SET v = v + {1: 'v1', 2: 'v2'} WHERE pk = 0 AND ck = 0;
APPLY BATCH;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v
--------------------------------------+----+----+--------------------+---------------
c72c7c3e-2fda-11eb-b78e-840de9f2b0dd | 0 | 0 | {1: 'v1', 2: 'v2'} | True
Non-frozen sets behave similarly to non-frozen maps. In fact, we can think of them as special cases of maps, where the type of the value is a unit type, i.e. a type with exactly one possible value. The only difference in CDC is the type of the of the added elements column in the CDC log table; instead of a frozen map, we’re using a frozen set.
For example:
CREATE TABLE ks.t (pk int, ck int, v set<int>, PRIMARY KEY (pk, ck)) WITH cdc = {'enabled': true};
DESCRIBE TABLE ks.t_scylla_cdc_log;
gives:
CREATE TABLE ks.t_scylla_cdc_log (
...
"cdc$deleted_elements_v" frozen<set<int>>,
"cdc$deleted_v" boolean,
...
v frozen<set<int>>,
...
For each column X set<K> in the base table, where X is a column name and K is a type, the CDC log table has 3 corresponding columns:
X frozen<set<K>>, the added elements column,
cdc$deleted_X boolean, the collection deletion column,
cdc$deleted_elements_X frozen<set<K>>, the deleted elements column.
The added elements column describes new elements added to the set:
UPDATE ks.t SET v = v + {1, 2} WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+--------+---------------+------------------------
d67d19b2-2fe1-11eb-d2a2-1f69144504ce | 0 | 0 | {1, 2} | null | null
The deleted elements column describes elements that are removed from the set:
UPDATE ks.t SET v = v - {1, 2, 3} WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+------+---------------+------------------------
eef36438-2fe1-11eb-b7b0-c0a03f64d719 | 0 | 0 | null | null | {1, 2, 3}
The collection deletion column is set to True when a collection-wide tombstone is created. Furthermore, a single UPDATE or INSERT statement can be used to overwrite a set using the = {...} notation, which is the same as creating a collection-wide tombstone and adding new elements at the same time. The behavior is the same as for maps. Example for deleting follows:
UPDATE ks.t SET v = null WHERE pk = 0 AND ck = 0;
UPDATE ks.t SET v = {} WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+------+---------------+------------------------
1c89ecc8-2fe2-11eb-f3d0-c8f925d280df | 0 | 0 | null | True | null
1c8a2850-2fe2-11eb-ad18-c31ba093947b | 0 | 0 | null | True | null
Another example for overwriting:
UPDATE ks.t SET v = {1, 2} WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+--------+---------------+------------------------
78a546ec-2fe2-11eb-3dda-ce195f6906c6 | 0 | 0 | {1, 2} | True | null
The rules for timestamps described in the Collection-wide tombstones and timestamps section apply.
Non-frozen lists are possibly the weirdest types you can find in ScyllaDB (and Cassandra). Perhaps it’s surprising when we say that non-frozen lists are also special cases of non-frozen maps; when querying tables that use lists, however, the key is hidden and only the values are shown. The type of the key used in the internal map representation of a list is timeuuid.
Although you can’t see list keys when using CQL read queries, you can update the value under any given key. For example:
CREATE TABLE ks.t (pk int, ck int, v list<int>, PRIMARY KEY (pk, ck));
UPDATE ks.t SET v[SCYLLA_TIMEUUID_LIST_INDEX(839e7120-2fe4-11eb-af55-000000000001)] = 0 WHERE pk = 0 AND ck = 0;
Thus, the syntax is:
UPDATE ... SET X[SCYLLA_TIMEUUID_LIST_INDEX(k)] = v WHERE ...;
where X is the list column name, k is a timeuuid, and v is a value of the list’s value type.
The keys define the order of elements in the list. When using the standard list update syntax (e.g. SET v = v + [1, 2]), the timeuuids are automatically generated by ScyllaDB using the current time. This method allows fast, conflict-free concurrent updates to the list (such as appending or prepending elements). This list representation is a simple example of a CRDT.
In the CDC log table, the key is revealed to the user. The design goals of CDC enforce this: the user should have the possibility of replaying the exact same sequence of changes to another ScyllaDB cluster in order to obtain the same result. In the case of lists, one can’t allow the other cluster to generate the timeuuids since the resulting order of elements in the list may end up different; the user must specify the keys on their own, thus they must be able to learn what the keys are in the first place.
Let’s start with an example:
CREATE TABLE ks.t (pk int, ck int, v list<int>, PRIMARY KEY (pk, ck)) WITH cdc = {'enabled': true};
DESCRIBE TABLE ks.t_scylla_cdc_log;
result:
CREATE TABLE ks.t_scylla_cdc_log (
...
"cdc$deleted_elements_v" frozen<set<timeuuid>>,
"cdc$deleted_v" boolean,
...
v frozen<map<timeuuid, int>>,
...
After reading the Maps section and the above discussion one shouldn’t be surprised by the resulting types. If one remembers that list<V> is syntactic sugar for map<timeuuid, V> everything else should become obvious.
For each column X list<V> in the base table, where X is a column name and V is a type, the CDC log table has 3 corresponding columns:
X frozen<map<timeuuid, V>>, the added elements column,
cdc$deleted_X boolean, the collection deletion column,
cdc$deleted_elements_X frozen<set<timeuuid>>, the deleted elements column.
The added elements column is filled when new elements are added to the list:
UPDATE ks.t SET v = v + [1, 2] WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+------------------------------------------------------------------------------------+---------------+------------------------
db320398-2fe9-11eb-6e4b-cb8fd582dbb5 | 0 | 0 | {db3214f0-2fe9-11eb-af55-000000000001: 1, db3214f1-2fe9-11eb-af55-000000000001: 2} | null | null
example of using the SCYLLA_TIMEUUID_LIST_INDEX syntax:
UPDATE ks.t SET v[SCYLLA_TIMEUUID_LIST_INDEX(0dd381f0-2fea-11eb-af55-000000000001)] = 0 WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+-------------------------------------------+---------------+------------------------
21c6a3fe-2fea-11eb-15ca-5fb980489b57 | 0 | 0 | {0dd381f0-2fea-11eb-af55-000000000001: 0} | null | null
as we can see, the added element’s key is equal to the timeuuid we’ve used in the UPDATE statement.
The deleted elements column describes elements that are removed from the list. ScyllaDB offers the v = v - [...] syntax for removing all elements whose values appear in the provided list of values. To do this, ScyllaDB first performs a read, obtaining the set of all keys that contain the values from the provided list, and then writes a tombstone for each obtained key. For example:
UPDATE ks.t SET v = v + [1, 2, 1, 3] WHERE pk = 0 AND ck = 0;
UPDATE ks.t SET v = v - [1] WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+----------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------+------------------------------------------------------------------------------
cc5bb172-2fec-11eb-2396-140c0c098ef0 | 0 | 0 | {cc5baec0-2fec-11eb-af55-000000000001: 1, cc5baec1-2fec-11eb-af55-000000000001: 2, cc5baec2-2fec-11eb-af55-000000000001: 1, cc5baec3-2fec-11eb-af55-000000000001: 3} | null | null
cc5bfc40-2fec-11eb-24e1-0da297c23c66 | 0 | 0 | null | null | {cc5baec0-2fec-11eb-af55-000000000001, cc5baec2-2fec-11eb-af55-000000000001}
We can see how the deleted keys in the second row are the same as the keys that were assigned to the list elements with value 1 in the first row. The resulting list is [2, 3], represented by the map {cc5baec1-2fec-11eb-af55-000000000001: 2, cc5baec3-2fec-11eb-af55-000000000001: 3} underneath.
You can also delete the value under a specific key:
UPDATE ks.t SET v[SCYLLA_TIMEUUID_LIST_INDEX(cc5baec0-2fec-11eb-af55-000000000001)] = null WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+------+---------------+----------------------------------------
23158f88-2fed-11eb-0968-538fd19b1920 | 0 | 0 | null | null | {cc5baec0-2fec-11eb-af55-000000000001}
The collection deletion column is set to True when a collection-wide tombstone is created. Furthermore, a single UPDATE or INSERT statement can be used to overwrite a list using the = [...] notation, which is the same as creating a collection-wide tombstone and adding new elements at the same time. The behavior is the same as for maps. Example for deleting:
UPDATE ks.t SET v = null WHERE pk = 0 AND ck = 0;
UPDATE ks.t SET v = [] WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+------+---------------+------------------------
fe29e712-2fee-11eb-ae33-9223dde04e80 | 0 | 0 | null | True | null
fe2a31e0-2fee-11eb-6a46-0da9bf86f6e9 | 0 | 0 | null | True | null
Another example for overwriting:
UPDATE ks.t SET v = [1, 2] WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+------------------------------------------------------------------------------------+---------------+------------------------
276b9724-2fef-11eb-d5b0-dbccd2164c64 | 0 | 0 | {276b8860-2fef-11eb-af55-000000000001: 1, 276b8861-2fef-11eb-af55-000000000001: 2} | True | null
The rules for timestamps described in the Collection-wide tombstones and timestamps section apply.
Unsurprisingly, non-frozen UDTs are also special cases of non-frozen maps. This time the key type is smallint and the keys stand for field indices. When using UDTs one refers to the field names and ScyllaDB translates them to keys (field indices) using schema internal definitions.
Understanding the correspondence between field names and field indices is important when using CDC with non-frozen UDT columns. Everything then works the same as for maps.
Let’s start with an example.
CREATE TYPE ks.ut (a int, b int, c int);
CREATE TABLE ks.t (pk int, ck int, v ut, PRIMARY KEY (pk, ck)) WITH cdc = {'enabled': true};
DESCRIBE TABLE ks.t_scylla_cdc_log;
result:
CREATE TABLE ks.t_scylla_cdc_log (
...
"cdc$deleted_elements_v" frozen<set<smallint>>,
"cdc$deleted_v" boolean,
...
v frozen<ut>,
...
For each column X T in the base table, where X is a column name and T is a user-defined-type, the CDC log table has 3 corresponding columns:
X frozen<T>, the added elements column,
cdc$deleted_X boolean, the collection deletion column,
cdc$deleted_elements_X frozen<set<smallint>>, the deleted elements column.
The added elements column is filled when non-null values are assigned to the fields of the user type:
UPDATE ks.t SET v.a = 0, v.b = 1 WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+-----------------------+---------------+------------------------
6380f682-2ff4-11eb-c312-0e51af7972af | 0 | 0 | {a: 0, b: 1, c: null} | null | null
All fields that were not assigned a non-null value are shown as null in the added elements column (in this case, c: null). Note that this is only an artifact of the CQL syntax and does not mean that the fields were deleted: field deletions are shown in the cdc$deleted_elements_X column, as we will see in the next subsection.
The deleted elements column is filled when fields are set to null:
UPDATE ks.t SET v.a = null, v.b = null WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+-----------------------------+---------------+------------------------
d5da679a-2ff4-11eb-6db1-fb8aacbc8bca | 0 | 0 | {a: null, b: null, c: null} | null | {0, 1}
as we can see, two field indices were shown in the cdc$deleted_elements_v column: the index 0 corresponds to field a, and the index 1 corresponds to field b. As explained in the Adding elements subsection above, since no field was assigned a non-null value, the v column shows each field as null (for no reason other than a CQL syntax quirk).
The indices are assigned to fields consecutively, starting from 0, in the order that the fields are listed in the result of DESCRIBE TYPE statement. For example, suppose that the following statement:
DESCRIBE TYPE ks.ut;
gives the following result:
CREATE TYPE ks.ut (
a int,
b int,
c int
);
The fields a, b, c are assigned indices 0, 1, 2 correspondingly. Renaming a field does not change its placement in the order, therefore it does not change the index that it’s assigned to. You can’t remove a field. You can only add a field, in which case the new field is assigned the next free index. Continuing the above example, suppose you execute the statement
ALTER TYPE ks.ut ADD d int;
The field d will be assigned index 3 (assuming there was no concurrent ALTER TYPE performed which executed before the above statement; use DESCRIBE TYPE to learn what the end result is).
The following example illustrates how the same statement can both set a non-null value to one of the fields and remove another:
UPDATE ks.t SET v.a = 42, v.c = null WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+---------------------------+---------------+------------------------
e0debf6e-2ff5-11eb-0509-e0877d00c0c2 | 0 | 0 | {a: 42, b: null, c: null} | null | {2}
The index 2 corresponds to field c, so the deleted elements set contains 2, as expected.
Unfortunately, ScyllaDB offers no syntax to operate directly on field indices; you can only perform writes using field names. Thus, to replay a CDC log entry which contains user type field deletions, you must manually translate the field indices to field names using the rule explained above.
The collection deletion column is set to True when a collection-wide tombstone is created. Furthermore, a single UPDATE or INSERT statement can be used to overwrite a set using the = {...} notation, which is the same as creating a collection-wide tombstone and adding new elements at the same time. The behavior is the same as for maps. Example for deleting follows:
UPDATE ks.t SET v = null WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+-----------------------------+---------------+------------------------
0176e250-2ff7-11eb-fcff-e94971544fd7 | 0 | 0 | {a: null, b: null, c: null} | True | null
Another example for overwriting:
UPDATE ks.t SET v = {a: 1, b: 2} WHERE pk = 0 AND ck = 0;
SELECT "cdc$time", pk, ck, v, "cdc$deleted_v", "cdc$deleted_elements_v" FROM ks.t_scylla_cdc_log;
result:
cdc$time | pk | ck | v | cdc$deleted_v | cdc$deleted_elements_v
--------------------------------------+----+----+-----------------------+---------------+------------------------
17a530b8-2ff7-11eb-ce58-8914310ad1e2 | 0 | 0 | {a: 1, b: 2, c: null} | True | null
The rules for timestamps described in the Collection-wide tombstones and timestamps section apply.
Counters are currently not supported in CDC. If you try to enable CDC on a table with counters, a message like the following will be generated:
Cannot create CDC log for table ks.t. Counter support not implemented.
You can track the counter support issue here: https://github.com/scylladb/scylla/issues/6013.
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