Fixing Patroni Continuous Restarts: Errors & Config

Hey there, fellow database enthusiasts! Ever found yourself staring at your Patroni logs, scratching your head as your PostgreSQL cluster keeps restarting? It's a frustrating situation, for sure, especially when your services are relying on that smooth, uninterrupted database flow. This article dives deep into a common scenario where a Patroni cluster, specifically node1 in our example, enters a continuous restart loop after a failover and switchback. We’re going to walk through the log output, pinpoint the root causes, and arm you with the knowledge to troubleshoot and resolve these pesky issues. We'll cover everything from mysterious FileNotFoundError messages to subtle configuration mismatches that can throw a wrench in your high-availability setup. So, buckle up, because we're about to demystify Patroni restarts and get your cluster back to tip-top shape!

Unpacking the Patroni Restart Saga: What Went Wrong?

So, your Patroni cluster, specifically node1, decided to throw a party of continuous restarts right after a leader failover to node2 and a subsequent switchback. This isn't just a minor hiccup; it signals a pretty significant problem with how Patroni is managing PostgreSQL on that particular node. When you see messages like "Postgresql is not running" repeatedly in your journalctl -u patroni output, combined with the alarming FileNotFoundError, it's a clear indicator that something fundamental has gone awry. Your patronictl list command might even show the other nodes as healthy replicas, making node1 the black sheep of the family. The core issue here isn't just that PostgreSQL isn't starting; it's why it can't start, and Patroni's relentless attempts to bring it up are what constitute the "continuous restarts." This situation demands immediate attention because a broken node impacts your cluster's resilience and capacity, especially in a two-node (or effectively two-node after one fails) setup, leaving you with reduced redundancy. The logs are our best friends here, acting as a crucial diagnostic tool to tell us the story of node1's struggles. Without understanding the specific errors, we'd be flying blind, simply rebooting and hoping for the best – which, as any seasoned admin knows, is rarely a viable long-term strategy for complex systems like Patroni and PostgreSQL. Therefore, a careful analysis of the provided log snippets is absolutely essential to correctly identify the underlying problems and formulate an effective solution to stabilize the cluster and ensure all nodes are functioning as expected.

The Critical FileNotFoundError: A Deep Dive into /postgres/pgdata/postgresql.conf

The most glaring and critical error leaping out from the Patroni logs is the FileNotFoundError: [Errno 2] No such file or directory: '/postgres/pgdata/postgresql.conf'. This isn't just a warning; it's a showstopper. Patroni, in its effort to start or reconfigure PostgreSQL, attempts to rename or modify the postgresql.conf file, which is absolutely vital for PostgreSQL to function. If this file is missing or inaccessible in the expected data directory (/postgres/pgdata/), PostgreSQL simply cannot start. Period. It's like trying to drive a car without an engine manual – you don't know how to operate it. Patroni's write_postgresql_conf function is responsible for dynamically generating or updating this configuration based on your Patroni YAML and the current cluster state. The os.rename operation failing with a FileNotFoundError strongly suggests one of two things: either postgresql.conf itself is genuinely missing from /postgres/pgdata, or Patroni is looking for it in the wrong place, or there's a permissions issue preventing Patroni from seeing or manipulating the file. This could happen if the data directory was partially wiped, corrupted, or if there was an incomplete cleanup after a previous failure or manual intervention. It's also possible that the path _postgresql_conf or _postgresql_base_conf that Patroni is trying to access or create/rename doesn't actually exist on the filesystem, or Patroni's user context doesn't have the necessary permissions. This error is fundamental because without a proper postgresql.conf, the PostgreSQL server executable (postgres) simply won't know how to initialize, where to find its data, or what parameters to use. Patroni, seeing PostgreSQL fail to start, will then retry, leading to the observed continuous restart loop. To resolve this, we need to ensure that the postgresql.conf file is present, correctly configured, and accessible within the /postgres/pgdata directory, or verify that Patroni's configuration points to the correct data directory where this crucial file resides. Without addressing this particular FileNotFoundError first, any other troubleshooting will likely be futile, as it's the primary blocker for PostgreSQL even attempting to come online. The integrity and presence of this configuration file are non-negotiable for a healthy PostgreSQL instance managed by Patroni.

Configuration Clash: max_worker_processes Mismatch

Beyond the critical file not found error, we've spotted another subtle but important detail in the Patroni logs: INFO: max_worker_processes value in pg_controldata: 8, in the global configuration: 4. This message, while not a hard error that stops PostgreSQL from starting, indicates a discrepancy between what Patroni believes max_worker_processes should be (4, based on your Patroni configuration under postgresql.parameters) and what the PostgreSQL control file (pg_controldata) on node1 actually reports (8). This mismatch can sometimes lead to unexpected behavior or performance issues, and in some cases, it can even prevent PostgreSQL from starting correctly if the values are drastically different or lead to resource conflicts. The pg_controldata output reflects the settings from the last successful initialization or shutdown of the PostgreSQL cluster, whereas Patroni's configuration is what it intends to apply. When Patroni starts PostgreSQL, it writes the postgresql.conf file based on its global configuration. If pg_controldata reports a different value for a parameter like max_worker_processes, it means that either Patroni hasn't successfully applied its desired configuration to this instance, or the instance was previously initialized with different settings that are now