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Memgraph vs. Adya weak consistency #807

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Jun 14, 2024
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Memgraph vs. Adya weak consistency #807

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Docs improvements (#799)
as51340 May 29, 2024
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Merge branch 'main' into memgraph-2-18
kgolubic May 29, 2024
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Add G0 knowledge
as51340 May 29, 2024
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PL-2 classification
as51340 Jun 3, 2024
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Isolation levels text
as51340 Jun 4, 2024
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Add G_single and G2_item phenomena
as51340 Jun 5, 2024
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Merge branch 'main' into adya-isolation-levels
kgolubic Jun 14, 2024
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18 changes: 17 additions & 1 deletion pages/fundamentals/transactions.mdx
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Expand Up @@ -280,13 +280,29 @@ lowest:
described in ANSI/ISO SQL-92 standard. In order to not mess up data consistency,
this isolation level should only be used in the read-only access mode.

| Phenomenon | Description | Isolation levels that protect against observing phenomenon |
| Phenomenon | Description | Disallowed by |
| ------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------- |
| Dirty Read | Transaction reads an object that was never committed by other transaction. | SNAPSHOT ISOLATION, READ COMMITTED |
| Non-repeatable Read | Transaction reads an object twice. When the txn reads the object 2nd time, it receives the modified value because other txn modified it. | SNAPSHOT ISOLATION |
| Phantom | Transaction reads objects meeting a certain condition and then finds additional objects when reading 2nd time because another txn added new objects. | SNAPHSOT ISOLATION |


Based on [Adya's classification of phenomena](https://pmg.csail.mit.edu/papers/adya-phd.pdf), we can further write:

| Phenomenon | Disallowed by |
| ---------- | ---------------------------------------------------- |
| G0 | SNAPHSOT ISOLATION, READ COMMITTED, READ UNCOMMITTED |
| G1a | SNAPSHOT ISOLATION, READ COMMITTED |
| G1b | SNAPSHOT ISOLATION, READ COMMITTED |
| G1c | SNAPSHOT ISOLATION, READ COMMITTED |
| G1-predA | SNAPSHOT ISOLATION, READ COMMITTED |
| G1-predB | SNAPSHOT ISOLATION, READ COMMITTED |
| G_single | SNAPSHOT ISOLATION |
| G2_item | NONE |

In terms of Adya's isolation levels, Memgraph supports: PL-1, PL-MSR (Monotonic Snapshot Reads), PL-2, PL-2', PL-2'', PL-2L, PL-CS (Cursor Stability) and PL-2+ (consistent view).

You can find tests for these phenomena [here](https://github.com/memgraph/memgraph/blob/master/tests/manual/test_isolation_level.py).

To check the current isolation level run the following query:

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