Scheduling reroute for delayed shards in delayed shards – How to solve this OpenSearch error

Opster Team

Aug-23, Version: 1-1.1

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Briefly, this error occurs when OpenSearch is unable to allocate shards to nodes immediately due to various reasons such as insufficient resources, node failures, or network issues. The system schedules a reroute to try and allocate these delayed shards later. To resolve this, you can increase the cluster’s resources, ensure the nodes are functioning properly, or check the network connectivity. Additionally, you can adjust the ‘cluster.routing.allocation.enable’ setting or the ‘index.unassigned.node_left.delayed_timeout’ to control the rerouting process.

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This guide will help you check for common problems that cause the log ” scheduling reroute for delayed shards in [{}] ({} delayed shards) ” to appear. To understand the issues related to this log, read the explanation below about the following OpenSearch concepts: shards, routing, cluster.

Log Context

Log “scheduling reroute for delayed shards in [{}] ({} delayed shards)” classname is
We extracted the following from OpenSearch source code for those seeking an in-depth context :

            } else {
                earlierRerouteNeeded = false;

            if (earlierRerouteNeeded) {
      "scheduling reroute for delayed shards in [{}] ({} delayed shards)"; nextDelay;
                DelayedRerouteTask currentTask = delayedRerouteTask.getAndSet(newTask);
                assert existingTask == currentTask || currentTask == null;
            } else {


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