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Advanced PostgreSQL: Indexing, Partitioning, Replication · レッスン

レプリケーションスロットとWAL管理

WALを確実に保持するレプリケーションスロットと、高度なWALファイル管理の技法を理解します。

「レプリケーションスロットとWAL管理」はCoddyKit上の無料Advanced PostgreSQL: Indexing, Partitioning, Replicationレッスンです。 これはレッスン3/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはAdvanced PostgreSQL: Indexing, Partitioning, Replication学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Advanced PostgreSQL: Indexing, Partitioning, Replicationコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

WAL: The Heart of PostgreSQL

At the core of PostgreSQL's reliability and replication lies the Write-Ahead Log (WAL). Think of WAL as a journal that records every change made to your database.

  • Every transaction writes to WAL before data is committed to disk.
  • This ensures data integrity, even if the server crashes.
  • Standby servers use WAL to reconstruct changes from the primary, staying in sync.

The WAL Retention Challenge

By default, PostgreSQL reclaims old WAL files to save disk space. This is fine for a standalone server, but it creates a challenge for replication.

If a standby server falls too far behind, the necessary WAL files might have already been removed from the primary. This leads to:

  • Replication failure: the standby can't catch up.
  • Manual intervention: requiring a full re-sync of the standby.

Introducing Replication Slots

Replication Slots are PostgreSQL's solution to the WAL retention problem. A replication slot is a persistent object on the primary server that prevents WAL segments required by a standby or logical decoder from being automatically removed.

  • They guarantee that WAL files are kept until consumed.
  • They track the progress of each connected consumer.
  • Slots ensure that a standby can always catch up, even after a long disconnection.

How Slots Ensure WAL Safety

When a replication slot is active, the primary server will not delete any WAL files that the slot's consumer (e.g., a standby server or a logical decoding process) has not yet confirmed as processed.

This creates a 'bookmark' in the WAL stream. The primary only purges WAL segments once all active slots have advanced past that point.

Creating a Physical Slot

Physical replication slots are used with physical streaming replication. They track the LSN (Log Sequence Number) of the standby, ensuring all necessary WAL files are retained for that standby.

To create a physical slot, you use the pg_create_physical_replication_slot() function. Replace my_physical_slot with a descriptive name.

SELECT pg_create_physical_replication_slot('my_physical_slot');

Creating a Logical Slot

Logical replication slots are used for logical replication (PostgreSQL's pub/sub model) or other logical decoding applications. They decode WAL into a stream of logical changes (e.g., INSERT, UPDATE, DELETE).

You specify a plugin (like pgoutput for built-in logical replication) to define how WAL records are transformed. Replace my_logical_slot with your desired name.

SELECT pg_create_logical_replication_slot('my_logical_slot', 'pgoutput');

Monitoring Replication Slots

It's crucial to monitor your replication slots to ensure they are active and not holding onto excessive WAL files. The pg_replication_slots view provides detailed information:

  • slot_name: The name of the slot.
  • active: True if a consumer is currently connected.
  • restart_lsn: The oldest WAL LSN still required by the slot.
  • confirmed_flush_lsn: For logical slots, the LSN confirmed by the consumer.

Try running this query to see existing slots:

SELECT
  slot_name,
  slot_type,
  active,
  restart_lsn,
  pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), restart_lsn)) AS wal_lag
FROM pg_replication_slots;

Dropping a Replication Slot

When a standby server or logical consumer is no longer needed, it's vital to drop its replication slot. An unused slot will continue to accumulate WAL files indefinitely, potentially filling up your primary server's disk space!

Use the pg_drop_replication_slot() function, providing the slot's name. Be careful: dropping an active slot will disconnect its consumer.

SELECT pg_drop_replication_slot('my_physical_slot');

Managing Disk Space & Inactive Slots

Replication slots are powerful, but they come with a responsibility: managing disk space. Inactive slots are a common cause of disk space exhaustion on primary servers.

  • Regularly check pg_replication_slots for inactive slots.
  • If a standby is permanently offline, drop its slot immediately.
  • Monitor the wal_lag (as shown in the monitoring query) to catch slow consumers.

Proactive management prevents outages!

Slot Management Quiz

You have a primary server and a standby that was decommissioned last week. The replication slot for this standby, named old_standby_slot, was not dropped. What is the most likely consequence for your primary server?

Recap: Replication Slots

In this lesson, we learned about PostgreSQL's Replication Slots, a critical feature for robust replication and logical decoding.

  • Slots guarantee WAL retention for connected consumers.
  • There are two types: physical for streaming replication and logical for logical decoding.
  • You can create, monitor (using pg_replication_slots), and critically, drop slots.
  • Always manage your slots carefully to prevent disk space issues caused by inactive slots!

よくある質問

「レプリケーションスロットとWAL管理」レッスンは無料ですか？

はい。「レプリケーションスロットとWAL管理」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Advanced PostgreSQL: Indexing, Partitioning, Replicationコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Advanced PostgreSQL: Indexing, Partitioning, Replicationコースには全4レッスンが含まれています。

「レプリケーションスロットとWAL管理」で何を学びますか？

WALを確実に保持するレプリケーションスロットと、高度なWALファイル管理の技法を理解します。 ブラウザで直接実行するハンズオンコードでAdvanced PostgreSQL: Indexing, Partitioning, Replicationを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Advanced PostgreSQL: Indexing, Partitioning, Replicationを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのAdvanced PostgreSQL: Indexing, Partitioning, Replicationは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン3/4です。

「レプリケーションスロットとWAL管理」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このAdvanced PostgreSQL: Indexing, Partitioning, Replicationレッスンでコードを書いて実行できますか？

はい。すべてのAdvanced PostgreSQL: Indexing, Partitioning, Replicationレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

  1. カスケードレプリケーション
  2. マルチマスターレプリケーション（BDR）
  3. レプリケーションスロットとWAL管理
  4. 論理デコーディングと変更データキャプチャ
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