# Common Errors

This page helps you understand and resolve error messages written to `stderr` or your logs.

Topic | Message
--- | ---
Client connection | `connection refused`
Client connection | `node is running secure mode, SSL connection required`
Transaction retries | `restart transaction`
Node startup | `node belongs to cluster <cluster ID> but is attempting to connect to a gossip network for cluster <another cluster ID>`
Node configuration | `clock synchronization error: this node is more than 500ms away from at least half of the known nodes`
Node configuration | `open file descriptor limit of <number> is under the minimum required <number>`
Replication | `replicas failing with "0 of 1 store with an attribute matching []; likely not enough nodes in cluster"`
Split failed | `split failed while applying backpressure; are rows updated in a tight loop?`
Deadline exceeded | `context deadline exceeded`
Ambiguous results | `result is ambiguous`

## connection refused

This message indicates a client is trying to connect to a node that is either not running or is not listening on the specified interfaces (i.e., hostname or port).

To resolve this issue, do one of the following:

- If the node hasn't yet been started, start the node.
- If you specified a `--listen-addr` and/or a `--advertise-addr` flag when starting the node, you must include the specified IP address/hostname and port with all other `cockroach` commands or change the `COCKROACH_HOST` environment variable.

If you're not sure what the IP address/hostname and port values might have been, you can look in the node's logs. If necessary, you can also terminate the `cockroach` process, and then restart the node:

- If the node was started with a process manager, gracefully stop the node by sending `SIGTERM` with the process manager. If the node is not shutting down after 1 minute, send `SIGKILL` to terminate the process. When using `systemd`, for example, set `TimeoutStopSec=60` in your configuration template and run `systemctl stop <systemd config filename>` to stop the node without `systemd` restarting it.
Note:

The amount of time you should wait before sending `SIGKILL` can vary depending on your cluster configuration and workload, which affects how long it takes your nodes to complete a graceful shutdown. In certain edge cases, forcefully terminating the process before the node has completed shutdown can result in temporary data unavailability, latency spikes, uncertainty errors, ambiguous commit errors, or query timeouts. If you need maximum cluster availability, you can run `cockroach node drain` prior to node shutdown and actively monitor the draining process instead of automating it.

- If the node was started using `cockroach start` and is running in the foreground, press `ctrl-c` in the terminal.
- If the node was started using `cockroach start` and the `--background` and `--pid-file` flags, run `kill <pid>`, where `<pid>` is the process ID of the node.

Then restart the node:

    $ cockroach start [flags]

## node is running secure mode, SSL connection required

This message indicates that the cluster is using TLS encryption to protect network communication, and the client is trying to open a connection without using the required TLS certificates.

To resolve this issue, use the `cockroach cert create-client` command to generate a client certificate and key for the user trying to connect. For a secure deployment walkthrough, including generating security certificates and connecting clients, see Manual Deployment.

## restart transaction

Messages with the error code `40001` and the string `restart transaction` indicate that a transaction failed because it conflicted with another concurrent or recent transaction accessing the same data. The transaction needs to be retried by the client. For more information about how to implement client-side retries, see client-side retry handling.

For more information about the different types of transaction retry errors such as "retry write too old", "read within uncertainty interval", etc., see the Transaction Retry Error Reference.

### read within uncertainty interval

(Error string includes: `ReadWithinUncertaintyIntervalError`)

Uncertainty errors can occur when two transactions which start on different gateway nodes attempt to operate on the same data at close to the same time. The uncertainty comes from the fact that we cannot tell which one started first - the clocks on the two gateway nodes may not be perfectly in sync.

For example, if the clock on node A is ahead of the clock on node B, a transaction started on node A may be able to commit a write with a timestamp that is still in the "future" from the perspective of node B. A later transaction that starts on node B should be able to see the earlier write from node A, even if B's clock has not caught up to A. The "read within uncertainty interval" occurs if we discover this situation in the middle of a transaction, when it is too late for the database to handle it automatically. When node B's transaction retries, it will unambiguously occur after the transaction from node A.

Note that as long as the client-side retry protocol is followed, a transaction that has restarted once is much less likely to hit another uncertainty error, and the `--max-offset` option provides an upper limit on how long a transaction can continue to restart due to uncertainty.

When errors like this occur, the application has the following options:

- Prefer consistent historical reads using AS OF SYSTEM TIME to reduce contention.
- Design the schema and queries to reduce contention. For information on how to avoid contention, see Understanding and Avoiding Transaction Contention.
- Be prepared to retry on uncertainty (and other) errors. For more information, see Transaction retries.

Note:

Uncertainty errors are a form of transaction conflict. For more information about transaction conflicts, see Transaction conflicts.

### transaction deadline exceeded

Errors which were previously reported to the client as opaque `TransactionStatusError`s are now transaction retry errors with the error message "transaction deadline exceeded" and error code `40001`.

This error can occur for long-running transactions (with execution time on the order of minutes) that also experience conflicts with other transactions and thus attempt to commit at a timestamp different than their original timestamp. If the timestamp at which the transaction attempts to commit is above a "deadline" imposed by the various schema elements that the transaction has used (i.e., table structures), then this error might get returned to the client.

When this error occurs, the application must retry the transaction. For more information about how to retry transactions, see Transaction retries.

Note:

For more information about the mechanics of the transaction conflict resolution process described above, see Life of a Distributed Transaction.

## node belongs to cluster <cluster ID> but is attempting to connect to a gossip network for cluster <another cluster ID>

This message usually indicates that a node tried to connect to a cluster, but the node is already a member of a different cluster. This is determined by metadata in the node's data directory. To resolve this issue, do one of the following:

- Choose a different directory to store the CockroachDB data:

      $ cockroach start [flags] --store=[new directory] --join=[cluster host]:26257

- Remove the existing directory and start a node joining the cluster again:

      $ rm -r cockroach-data/

      $ cockroach start [flags] --join=[cluster host]:26257

## clock synchronization error: this node is more than 500ms away from at least half of the known nodes

This error indicates that a node has spontaneously shut down because it detected that its clock is out of sync with at least half of the other nodes in the cluster by 80% of the maximum offset allowed (500ms by default). CockroachDB requires moderate levels of clock synchronization to preserve data consistency, so the node shutting down in this way avoids the risk of consistency anomalies.

To prevent this from happening, you should run clock synchronization software on each node. For guidance on synchronizing clocks, see the tutorial for your deployment environment:

Environment | Recommended Approach
--- | ---
Manual | Use NTP with Google's external NTP service.
AWS | Use the Amazon Time Sync Service.
Azure | Disable Hyper-V time synchronization and use NTP with Google's external NTP service.
Digital Ocean | Use NTP with Google's external NTP service.
GCE | Use NTP with Google's internal NTP service.

## open file descriptor limit of <number> is under the minimum required <number>

CockroachDB can use a large number of open file descriptors, often more than is available by default. This message indicates that the machine on which a CockroachDB node is running is under CockroachDB's recommended limits.

For more details on CockroachDB's file descriptor limits and instructions on increasing the limit on various platforms, see File Descriptors Limit.

## replicas failing with "0 of 1 store with an attribute matching []; likely not enough nodes in cluster

### When running a single-node cluster

When running a single-node CockroachDB cluster, an error about replicas failing will eventually show up in the node's log files, for example:

    E160407 09:53:50.337328 storage/queue.go:511  [replicate] 7 replicas failing with "0 of 1 store with an attribute matching []; likely not enough nodes in cluster"

This happens because CockroachDB expects three nodes by default. If you do not intend to add additional nodes, you can stop this error by using `ALTER RANGE ... CONFIGURE ZONE` to update your default zone configuration to expect only one node:

    # Insecure cluster:
    $ cockroach sql --execute="ALTER RANGE default CONFIGURE ZONE USING num_replicas=1;" --insecure

    # Secure cluster:
    $ cockroach sql --execute="ALTER RANGE default CONFIGURE ZONE USING num_replicas=1;" --certs-dir=[path to certs directory]

The zone's replica count is reduced to 1. For more information, see `ALTER RANGE ... CONFIGURE ZONE` and Configure Replication Zones.

### When running a multi-node cluster

When running a multi-node CockroachDB cluster, if you see an error like the one above about replicas failing, some nodes might not be able to talk to each other. For recommended actions, see Cluster Setup Troubleshooting.

## split failed while applying backpressure; are rows updated in a tight loop?

In CockroachDB, a table row is stored on disk as a key-value pair. Whenever the row is updated, CockroachDB also stores a distinct version of the key-value pair to enable concurrent request processing while guaranteeing consistency (see multi-version concurrency control (MVCC)). All versions of a key-value pair belong to a larger "range" of the total key space, and the historical versions remain until the garbage collection period defined by the `gc.ttlseconds` variable in the applicable zone configuration has passed (25 hours by default). Once a range reaches a size threshold (512 MiB by default), CockroachDB splits the range into two ranges. However, this message indicates that a range cannot be split as intended.

One possible cause is that the range consists only of MVCC version data due to a row being repeatedly updated, and the range cannot be split because doing so would spread MVCC versions for a single row across multiple ranges.

To resolve this issue, make sure you are not repeatedly updating a single row. If frequent updates of a row are necessary, consider one of the following:

- Reduce the `gc.ttlseconds` variable in the applicable zone configuration to reduce the garbage collection period and prevent such a large build-up of historical values.
- If a row contains large columns that are not being updated with other columns, put the large columns in separate column families.

## context deadline exceeded

This message occurs when a component of CockroachDB gives up because it was relying on another component that has not behaved as expected, for example, another node dropped a network connection. To investigate further, look in the node's logs for the primary failure that is the root cause.

## result is ambiguous

In a distributed system, some errors can have ambiguous results. For example, if you receive a `connection closed` error while processing a `COMMIT` statement, you cannot tell whether the transaction successfully committed or not. These errors are possible in any database, but CockroachDB is somewhat more likely to produce them than other databases because ambiguous results can be caused by failures between the nodes of a cluster. These errors are reported with the PostgreSQL error code `40003` (`statement_completion_unknown`) and the message `result is ambiguous`.

Ambiguous errors can be caused by nodes crashing, network failures, or timeouts. If you experience a lot of these errors when things are otherwise stable, look for performance issues. Note that ambiguity is only possible for the last statement of a transaction (`COMMIT` or `RELEASE SAVEPOINT`) or for statements outside a transaction. If a connection drops during a transaction that has not yet tried to commit, the transaction will definitely be aborted.

In general, you should handle ambiguous errors the same way as `connection closed` errors. If your transaction is idempotent, it is safe to retry it on ambiguous errors. `UPSERT` operations are typically idempotent, and other transactions can be written to be idempotent by verifying the expected state before performing any writes. Increment operations such as `UPDATE my_table SET x=x+1 WHERE id=$1` are typical examples of operations that cannot easily be made idempotent. If your transaction is not idempotent, then you should decide whether to retry or not based on whether it would be better for your application to apply the transaction twice or return an error to the user.

## Something else?

Try searching the rest of our docs for answers or using our other support resources, including:

- CockroachDB Community Forum
- CockroachDB Community Slack
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- CockroachDB Support Portal