Implemented CRDTs

Implemented CRDTs#

Currently, six CRDT data types are implemented:

  • Grow-only counter and sum

  • Positive-negative counter and sum

  • Delta counter and sum

The counters and sums behave mostly the same, except that the counter types are integer based (bigint ), while the sum types are decimal-based (numeric ).

You can list the currently implemented CRDT data types with the following query:

SELECT n.nspname, t.typname
FROM bdr.crdt_handlers c
JOIN (pg_type t JOIN pg_namespace n ON t.typnamespace = n.oid)
  ON t.oid = c.crdt_type_id;

Grow-only counter (crdt_gcounter )#

  • Supports only increments with nonnegative values (value + int and counter + bigint operators).

  • You can obtain the current value of the counter either using # operator or by casting it to bigint .

  • Isn’t compatible with simple assignments like counter = value (which is common pattern when the new value is computed somewhere in the application).

  • Allows simple reset of the counter using the ! operator ( counter = !counter ).

  • You can inspect the internal state using crdt_gcounter_to_text .

CREATE TABLE crdt_test (
    id       INT PRIMARY KEY,
    cnt      bdr.crdt_gcounter NOT NULL DEFAULT 0
);

INSERT INTO crdt_test VALUES (1, 0);      -- initialized to 0
INSERT INTO crdt_test VALUES (2, 129824); -- initialized to 129824
INSERT INTO crdt_test VALUES (3, -4531);  -- error: negative value

- - enable CLCD on the table
ALTER TABLE crdt_test REPLICA IDENTITY FULL;
SELECT bdr.alter_table_conflict_detection(crdt_test, column_modify_timestamp, cts);

- - increment counters
UPDATE crdt_test SET cnt = cnt + 1 WHERE id = 1;
UPDATE crdt_test SET cnt = cnt + 120 WHERE id = 2;

- - error: minus operator not defined
UPDATE crdt_test SET cnt = cnt - 1 WHERE id = 1;

- - error: increment has to be non-negative
UPDATE crdt_test SET cnt = cnt + (-1) WHERE id = 1;

- - reset counter
UPDATE crdt_test SET cnt = !cnt WHERE id = 1;

- - get current counter value
SELECT id, cnt::bigint, cnt FROM crdt_test;

- - show internal structure of counters
SELECT id, bdr.crdt_gcounter_to_text(cnt) FROM crdt_test;

Grow-only sum (crdt_gsum )#

  • Supports only increments with nonnegative values (sum + numeric ).

  • You can obtain the current value of the sum either by using the # operator or by casting it to numeric .

  • Isn’t compatible with simple assignments like sum = value , which is the common pattern when the new value is computed somewhere in the application.

  • Allows simple reset of the sum using the ! operator (sum = !sum ).

  • Can inspect internal state using crdt_gsum_to_text .

CREATE TABLE crdt_test (
    id       INT PRIMARY KEY,
    gsum     bdr.crdt_gsum NOT NULL DEFAULT 0.0
);

INSERT INTO crdt_test VALUES (1, 0.0);      -- initialized to 0
INSERT INTO crdt_test VALUES (2, 1298.24); -- initialized to 1298.24
INSERT INTO crdt_test VALUES (3, -45.31);  -- error: negative value

- - enable CLCD on the table
ALTER TABLE crdt_test REPLICA IDENTITY FULL;
SELECT bdr.alter_table_conflict_detection(crdt_test, column_modify_timestamp, cts);

- - increment sum
UPDATE crdt_test SET gsum = gsum + 11.5 WHERE id = 1;
UPDATE crdt_test SET gsum = gsum + 120.33 WHERE id = 2;

- - error: minus operator not defined
UPDATE crdt_test SET gsum = gsum - 15.2 WHERE id = 1;

- - error: increment has to be non-negative
UPDATE crdt_test SET gsum = gsum + (-1.56) WHERE id = 1;

- - reset sum
UPDATE crdt_test SET gsum = !gsum WHERE id = 1;

- - get current sum value
SELECT id, gsum::numeric, gsum FROM crdt_test;

- - show internal structure of sums
SELECT id, bdr.crdt_gsum_to_text(gsum) FROM crdt_test;

Positive-negative counter (crdt_pncounter )#

  • Supports increments with both positive and negative values (through counter + int and counter + bigint operators).

  • You can obtain the current value of the counter either by using the # operator or by casting to bigint .

  • Isn’t compatible with simple assignments like counter = value , which is the common pattern when the new value is computed somewhere in the application.

  • Allows simple reset of the counter using the ! operator (counter = !counter ).

  • You can inspect the internal state using crdt_pncounter_to_text .

CREATE TABLE crdt_test (
    id       INT PRIMARY KEY,
    cnt      bdr.crdt_pncounter NOT NULL DEFAULT 0
);

INSERT INTO crdt_test VALUES (1, 0);      -- initialized to 0
INSERT INTO crdt_test VALUES (2, 129824); -- initialized to 129824
INSERT INTO crdt_test VALUES (3, -4531);  -- initialized to -4531

- - enable CLCD on the table
ALTER TABLE crdt_test REPLICA IDENTITY FULL;
SELECT bdr.alter_table_conflict_detection(crdt_test, column_modify_timestamp, cts);

- - increment counters
UPDATE crdt_test SET cnt = cnt + 1      WHERE id = 1;
UPDATE crdt_test SET cnt = cnt + 120    WHERE id = 2;
UPDATE crdt_test SET cnt = cnt + (-244) WHERE id = 3;

- - decrement counters
UPDATE crdt_test SET cnt = cnt - 73    WHERE id = 1;
UPDATE crdt_test SET cnt = cnt - 19283 WHERE id = 2;
UPDATE crdt_test SET cnt = cnt - (-12) WHERE id = 3;

- - get current counter value
SELECT id, cnt::bigint, cnt FROM crdt_test;

- - show internal structure of counters
SELECT id, bdr.crdt_pncounter_to_text(cnt) FROM crdt_test;

- - reset counter
UPDATE crdt_test SET cnt = !cnt WHERE id = 1;

- - get current counter value after the reset
SELECT id, cnt::bigint, cnt FROM crdt_test;

Positive-negative sum (crdt_pnsum )#

  • Supports increments with both positive and negative values through sum + numeric .

  • You can obtain the current value of the sum either by using then # operator or by casting to numeric .

  • Isn’t compatible with simple assignments like sum = value , which is the common pattern when the new value is computed somewhere in the application.

  • Allows simple reset of the sum using the ! operator (sum = !sum ).

  • You can inspect the internal state using crdt_pnsum_to_text .

CREATE TABLE crdt_test (
    id       INT PRIMARY KEY,
    pnsum    bdr.crdt_pnsum NOT NULL DEFAULT 0
);

INSERT INTO crdt_test VALUES (1, 0);       -- initialized to 0
INSERT INTO crdt_test VALUES (2, 1298.24); -- initialized to 1298.24
INSERT INTO crdt_test VALUES (3, -45.31);  -- initialized to -45.31

- - enable CLCD on the table
ALTER TABLE crdt_test REPLICA IDENTITY FULL;
SELECT bdr.alter_table_conflict_detection(crdt_test, column_modify_timestamp, cts);

- - increment sums
UPDATE crdt_test SET pnsum = pnsum + 1.44     WHERE id = 1;
UPDATE crdt_test SET pnsum = pnsum + 12.20    WHERE id = 2;
UPDATE crdt_test SET pnsum = pnsum + (-24.34) WHERE id = 3;

- - decrement sums
UPDATE crdt_test SET pnsum = pnsum - 7.3      WHERE id = 1;
UPDATE crdt_test SET pnsum = pnsum - 192.83   WHERE id = 2;
UPDATE crdt_test SET pnsum = pnsum - (-12.22) WHERE id = 3;

- - get current sum value
SELECT id, pnsum::numeric, pnsum FROM crdt_test;

- - show internal structure of sum
SELECT id, bdr.crdt_pnsum_to_text(pnsum) FROM crdt_test;

- - reset sum
UPDATE crdt_test SET pnsum = !pnsum WHERE id = 1;

- - get current sum value after the reset
SELECT id, pnsum::numeric, pnsum FROM crdt_test;

Delta counter (crdt_delta_counter )#

  • Is defined a bigint domain, so works exactly like a bigint column.

  • Supports increments with both positive and negative values.

  • Is compatible with simple assignments like counter = value , which is common when the new value is computed somewhere in the application.

  • There’s no simple way to reset the value reliably.

CREATE TABLE crdt_test (
    id       INT PRIMARY KEY,
    cnt      bdr.crdt_delta_counter NOT NULL DEFAULT 0
);

INSERT INTO crdt_test VALUES (1, 0);      -- initialized to 0
INSERT INTO crdt_test VALUES (2, 129824); -- initialized to 129824
INSERT INTO crdt_test VALUES (3, -4531);  -- initialized to -4531

- - enable CLCD on the table
ALTER TABLE crdt_test REPLICA IDENTITY FULL;
SELECT bdr.alter_table_conflict_detection(crdt_test, column_modify_timestamp, cts);

- - increment counters
UPDATE crdt_test SET cnt = cnt + 1      WHERE id = 1;
UPDATE crdt_test SET cnt = cnt + 120    WHERE id = 2;
UPDATE crdt_test SET cnt = cnt + (-244) WHERE id = 3;

- - decrement counters
UPDATE crdt_test SET cnt = cnt - 73    WHERE id = 1;
UPDATE crdt_test SET cnt = cnt - 19283 WHERE id = 2;
UPDATE crdt_test SET cnt = cnt - (-12) WHERE id = 3;

- - get current counter value
SELECT id, cnt FROM crdt_test;

Delta sum (crdt_delta_sum )#

  • Is defined as a numeric domain so works exactly like a numeric column.

  • Supports increments with both positive and negative values.

  • Is compatible with simple assignments like sum = value , which is common when the new value is computed somewhere in the application.

  • There’s no simple way to reset the value reliably.

CREATE TABLE crdt_test (
    id       INT PRIMARY KEY,
    dsum     bdr.crdt_delta_sum NOT NULL DEFAULT 0
);

INSERT INTO crdt_test VALUES (1, 0);       -- initialized to 0
INSERT INTO crdt_test VALUES (2, 129.824); -- initialized to 129824
INSERT INTO crdt_test VALUES (3, -4.531);  -- initialized to -4531

- - enable CLCD on the table
ALTER TABLE crdt_test REPLICA IDENTITY FULL;
SELECT bdr.alter_table_conflict_detection(crdt_test, column_modify_timestamp, cts);

- - increment counters
UPDATE crdt_test SET dsum = dsum + 1.32   WHERE id = 1;
UPDATE crdt_test SET dsum = dsum + 12.01  WHERE id = 2;
UPDATE crdt_test SET dsum = dsum + (-2.4) WHERE id = 3;

- - decrement counters
UPDATE crdt_test SET dsum = dsum - 7.33   WHERE id = 1;
UPDATE crdt_test SET dsum = dsum - 19.83  WHERE id = 2;
UPDATE crdt_test SET dsum = dsum - (-1.2) WHERE id = 3;

- - get current counter value
SELECT id, cnt FROM crdt_test;
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