Contents

• JSONCDC
  • Copyright and License
  • Status
  • Usage

JSONCDC

JSONCDC provides change data capture for Postgres, translating the Postgres write ahead log to JSON.

It is written in Rust and, being short, is a good skeleton project for other would be plugin authors who'd like to use Rust to write Postgres extensions.

Our library Requires rust stable 1.1 or greater.

Copyright and License

Copyright (c) 2016 Alex Newman, Jason Dusek Copyright (c) 2018 Instructure, Inc.

JSONCDC is available under multiple licenses:

• the same license as Postgres itself (licenses/postgres),

• the Apache 2.0 license (licenses/apache).

Status

JSONCDC is presently installable with pgxn, from the testing channel: pgxn install jsoncdc --testing.

Usage

A basic demo:

SELECT * FROM pg_create_logical_replication_slot('jsoncdc', 'jsoncdc');
--- Wait for some transactions, and then:
SELECT * FROM pg_logical_slot_get_changes('jsoncdc', NULL, NULL);

The output format of jsoncdc is very regular, consisting of begin, table, insert, update, delete and message clauses as JSON objects, one per line:

{ "begin": <xid> }
{ "schema": <column names and type>, "table": <name of table> }
...inserts, updates and deletes for this table...
{ "schema": <column names and type>, "table": <name of next table> }
...inserts, updates and deletes for next table...
{ "prefix": <prefix>, "message": <message>, "transactional": <true|false> }
...messages may be mixed in at any point; they don't belong to a table...
{ "commit": <xid>, "t": <timestamp with timezone> }

With pg_recvlogical and a little shell, you can leverage this very regular formatting to get each transaction batched into a separate file:

pg_recvlogical -S jsoncdc -d postgres:/// --start -f - |
while read -r line
do
  case "$line" in
    '{ "begin": '*)                # Close and reopen FD 9 for each new XID
      fields=( $line )
      xid="${fields[2]}"
      exec 9>&-
      exec 9> "txn-${xid}.json" ;;
  esac
  printf '%s\n' "$line" >&9       # Use printf because echo is non-portable
done