[RND-643] Serializable Snapshot Isolation (SSI) in PostgreSQL

Add documentation about PostgreSQL Serializable Snapshot Isolation (SSI) performance testing
Add a note to locking\delete.test.ts

Author: jleiva-gap

Meadowlark-js/backends/meadowlark-postgresql-backend/test/integration/locking/Delete.test.ts

@@ -3,6 +3,10 @@
 // The Ed-Fi Alliance licenses this file to you under the Apache License, Version 2.0.
 // See the LICENSE and NOTICES files in the project root for more information.
 
+/**
+ * NOTE: If Postgres query transactions are modified to use SSI and the use of SELECT...FOR UPDATE/SHARE is removed,
+ * this test case should be updated to adjust its logic and avoid deadlocks at the database level.
+ */
 import {
   DocumentInfo,
   NoDocumentInfo,

docs/performance-testing/postgres-SSI-performance-testing.md

@@ -0,0 +1,57 @@
+# RND-643: PostgreSQL Serializable Snapshot Isolation (SSI) performance testing
+
+## Goal
+
+Run Postgresql performance to compare the performance of the SSI implementation vs the SELECT...FOR UPDATE/SHARE implementation.
+
+## Description
+
+To test performance we use Autocannon tool.
+
+Changed transaction usage to use SSI. Additionally, retry logic was added with this change. With this logic, if a 40001 error occurs, a retry is executed. All functions that use transactions have been modified.
+
+As a side effect, the crash test\delete.test.ts failed. This test has transactions to create a lock and has no retry mechanism, so the test creates a deadlock and returns a timeout error.
+
+To capture database statistics we are reading data from pg_stat_database. A process runs each 5 seconds and inserts a row with the current statistics. With all the rows inserted, we can get an average of some indicators. From pg_stat_database we can get:
+
+- xact_commit: number of transactions in the database that have been completed successfully.
+- xact_rollback: number of transactions that have been cancelled or rolled back.
+- blks_read: number of disk blocks that have been read directly from the disk
+- blks_hit: This is the number of times that the needed data was found in the cache or buffer.
+- tup_returned: number of rows returned by queries in the database
+- tup_fetched: number of rows fetched by queries in the database
+- tup_inserted: number of rows inserted
+- tup_updated: number of rows updated
+- tup_deleted: number of rows deleted
+
+## Postgres Comparison
+
+For testing, Autocannon was run in the existing version in main and in the modified version. For each test, 3 iterations were executed and the comparison was made with the data obtained.
+
+First, we tested Autocannon with the original version and the modified version.
+
+||2xxx|Non-2xxx|Latency|Request|Throughput
+| :--- | ---: | ---: | ---: | ---: | ---: |
+**Non-SSI**|7075.67|0|564.35|44.51|15532.14
+**SSI**|5723.33|8659.66666666667|277.82|90.28|28020.23
+**Differences**|**-19.11%**|**100.00%**|**-50.77%**|**102.83%**|**80.40%**
+
+For this case we can see:
+
+- The SSI version has less 2xxx responses and more Non-2xxx. Now, we this change we are not locking the database, so that behavior is expected.
+- In the same case, the SSI version has less latency and more requests and throughput compared to the original version.
+
+With this results we can see some improvement in the performance.
+
+Now, if we compare database statistics, we have the following results:
+
+|Pool Size | Avg commit | Avg rollback | Avg disk blocks read | Avg blocks hit cache | Avg tuples returned | Avg tuples fetched | Avg tuples_inserted | Avg tuples updated | Avg tuples deleted |
+| :--- | ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: |
+Non-SSI|328.20|0|30.52|7662.52|1432.53|788.24|327.96|73.20|281.24
+SSI|149.51|98.88|17.03|5955.38|4748.24|445.19|151.52|46.46|153.13
+|**Differences**|**-54.00%**|**100.00%**|**-44.00%**|**-22.00%**|**231.00%**|**-44.00%**|**-54.00%**|**-37.00%**|**-46.00%**
+
+These results are consistent with the Autocannon results:
+
+- SSI version has less commits than previous version, and more rollbacks than previous version, that makes sense because we are not locking the database and we are executing a rollback to retry.
+- Also, SSI version returned more tuples but affected tuples are less compared to previous version. That behavior is consistent with a retry mechanism, because when we need to retry, we need to restart the process.