host.json reference for Azure Functions

The host.json metadata file contains global configuration options that affect all functions for a function app. This article lists the settings that are available. The JSON schema is at http://json.schemastore.org/host.

There are other global configuration options in app settings and in the local.settings.json file.

Sample host.json file

The following sample host.json file has all possible options specified.

{
    "aggregator": {
        "batchSize": 1000,
        "flushTimeout": "00:00:30"
    },
    "applicationInsights": {
        "sampling": {
          "isEnabled": true,
          "maxTelemetryItemsPerSecond" : 5
        }
    },
    "eventHub": {
      "maxBatchSize": 64,
      "prefetchCount": 256,
      "batchCheckpointFrequency": 1
    },
    "functions": [ "QueueProcessor", "GitHubWebHook" ],
    "functionTimeout": "00:05:00",
    "healthMonitor": {
        "enabled": true,
        "healthCheckInterval": "00:00:10",
        "healthCheckWindow": "00:02:00",
        "healthCheckThreshold": 6,
        "counterThreshold": 0.80
    },
    "http": {
        "routePrefix": "api",
        "maxOutstandingRequests": 20,
        "maxConcurrentRequests": 10,
        "dynamicThrottlesEnabled": false
    },
    "id": "9f4ea53c5136457d883d685e57164f08",
    "logger": {
        "categoryFilter": {
            "defaultLevel": "Information",
            "categoryLevels": {
                "Host": "Error",
                "Function": "Error",
                "Host.Aggregator": "Information"
            }
        }
    },
    "queues": {
      "maxPollingInterval": 2000,
      "visibilityTimeout" : "00:00:30",
      "batchSize": 16,
      "maxDequeueCount": 5,
      "newBatchThreshold": 8
    },
    "serviceBus": {
      "maxConcurrentCalls": 16,
      "prefetchCount": 100,
      "autoRenewTimeout": "00:05:00"
    },
    "singleton": {
      "lockPeriod": "00:00:15",
      "listenerLockPeriod": "00:01:00",
      "listenerLockRecoveryPollingInterval": "00:01:00",
      "lockAcquisitionTimeout": "00:01:00",
      "lockAcquisitionPollingInterval": "00:00:03"
    },
    "tracing": {
      "consoleLevel": "verbose",
      "fileLoggingMode": "debugOnly"
    },
    "watchDirectories": [ "Shared" ],
}

The following sections of this article explain each top-level property. All are optional unless otherwise indicated.

aggregator

Specifies how many function invocations are aggregated when calculating metrics for Application Insights.

{
    "aggregator": {
        "batchSize": 1000,
        "flushTimeout": "00:00:30"
    }
}
Property Default Description
batchSize 1000 Maximum number of requests to aggregate.
flushTimeout 00:00:30 Maximum time period to aggregate.

Function invocations are aggregated when the first of the two limits are reached.

applicationInsights

Controls the sampling feature in Application Insights.

{
    "applicationInsights": {
        "sampling": {
          "isEnabled": true,
          "maxTelemetryItemsPerSecond" : 5
        }
    }
}
Property Default Description
isEnabled true Enables or disables sampling.
maxTelemetryItemsPerSecond 5 The threshold at which sampling begins.

eventHub

Configuration settings for Event Hub triggers and bindings.

{
    "eventHub": {
      "maxBatchSize": 64,
      "prefetchCount": 256,
      "batchCheckpointFrequency": 1
    }
}
Property Default Description
maxBatchSize 64 The maximum event count received per receive loop.
prefetchCount n/a The default PrefetchCount that will be used by the underlying EventProcessorHost.
batchCheckpointFrequency 1 The number of event batches to process before creating an EventHub cursor checkpoint.

functions

A list of functions that the job host will run. An empty array means run all functions. Intended for use only when running locally. In function apps, use the function.json disabled property rather than this property in host.json.

{
    "functions": [ "QueueProcessor", "GitHubWebHook" ]
}

functionTimeout

Indicates the timeout duration for all functions. In Consumption plans, the valid range is from 1 second to 10 minutes, and the default value is 5 minutes. In App Service plans, there is no limit and the default value is null, which indicates no timeout.

{
    "functionTimeout": "00:05:00"
}

healthMonitor

Configuration settings for Host health monitor.

{
    "healthMonitor": {
        "enabled": true,
        "healthCheckInterval": "00:00:10",
        "healthCheckWindow": "00:02:00",
        "healthCheckThreshold": 6,
        "counterThreshold": 0.80
    }
}
Property Default Description
enabled true Whether the feature is enabled.
healthCheckInterval 10 seconds The time interval between the periodic background health checks.
healthCheckWindow 2 minutes A sliding time window used in conjunction with the healthCheckThreshold setting.
healthCheckThreshold 6 Maximum number of times the health check can fail before a host recycle is initiated.
counterThreshold 0.80 The threshold at which a performance counter will be considered unhealthy.

http

Configuration settings for http triggers and bindings.

{
    "http": {
        "routePrefix": "api",
        "maxOutstandingRequests": 20,
        "maxConcurrentRequests": 10,
        "dynamicThrottlesEnabled": false
    }
}
Property Default Description
routePrefix api The route prefix that applies to all routes. Use an empty string to remove the default prefix.
maxOutstandingRequests -1 The maximum number of outstanding requests that are held at any given time. This limit includes requests that are queued but have not started executing, as well as any in progress executions. Any incoming requests over this limit are rejected with a 429 "Too Busy" response. That allows callers to employ time-based retry strategies, and also helps you to control maximum request latencies. This only controls queuing that occurs within the script host execution path. Other queues such as the ASP.NET request queue will still be in effect and unaffected by this setting. The default is unbounded.
maxConcurrentRequests -1 The maximum number of http functions that will be executed in parallel. This allows you to control concurrency, which can help manage resource utilization. For example, you might have an http function that uses a lot of system resources (memory/cpu/sockets) such that it causes issues when concurrency is too high. Or you might have a function that makes outbound requests to a third party service, and those calls need to be rate limited. In these cases, applying a throttle here can help. The default is unbounded.
dynamicThrottlesEnabled false When enabled, this setting causes the request processing pipeline to periodically check system performance counters like connections/threads/processes/memory/cpu/etc. and if any of those counters are over a built-in high threshold (80%), requests will be rejected with a 429 "Too Busy" response until the counter(s) return to normal levels.

id

The unique ID for a job host. Can be a lower case GUID with dashes removed. Required when running locally. When running in Azure Functions, an ID is generated automatically if id is omitted.

If you share a Storage account across multiple function apps, make sure that each function app has a different id. You can omit the id property or manually set each function app's id to a different value. The timer trigger uses a storage lock to ensure that there will be only one timer instance when a function app scales out to multiple instances. If two function apps share the same id and each uses a timer trigger, only one timer will run.

{
    "id": "9f4ea53c5136457d883d685e57164f08"
}

logger

Controls filtering for logs written by an ILogger object or by context.log.

{
    "logger": {
        "categoryFilter": {
            "defaultLevel": "Information",
            "categoryLevels": {
                "Host": "Error",
                "Function": "Error",
                "Host.Aggregator": "Information"
            }
        }
    }
}
Property Default Description
categoryFilter n/a Specifies filtering by category
defaultLevel Information For any categories not specified in the categoryLevels array, send logs at this level and above to Application Insights.
categoryLevels n/a An array of categories that specifies the minimum log level to send to Application Insights for each category. The category specified here controls all categories that begin with the same value, and longer values take precedence. In the preceding sample host.json file, all categories that begin with "Host.Aggregator" log at Information level. All other categories that begin with "Host", such as "Host.Executor", log at Error level.

queues

Configuration settings for Storage queue triggers and bindings.

{
    "queues": {
      "maxPollingInterval": 2000,
      "visibilityTimeout" : "00:00:30",
      "batchSize": 16,
      "maxDequeueCount": 5,
      "newBatchThreshold": 8
    }
}
Property Default Description
maxPollingInterval 60000 The maximum interval in milliseconds between queue polls.
visibilityTimeout 0 The time interval between retries when processing of a message fails.
batchSize 16 The number of queue messages that the Functions runtime retrieves simultaneously and processes in parallel. When the number being processed gets down to the newBatchThreshold, the runtime gets another batch and starts processing those messages. So the maximum number of concurrent messages being processed per function is batchSize plus newBatchThreshold. This limit applies separately to each queue-triggered function.

If you want to avoid parallel execution for messages received on one queue, you can set batchSize to 1. However, this setting eliminates concurrency only so long as your function app runs on a single virtual machine (VM). If the function app scales out to multiple VMs, each VM could run one instance of each queue-triggered function.

The maximum batchSize is 32.
maxDequeueCount 5 The number of times to try processing a message before moving it to the poison queue.
newBatchThreshold batchSize/2 Whenever the number of messages being processed concurrently gets down to this number, the runtime retrieves another batch.

serviceBus

Configuration setting for Service Bus triggers and bindings.

{
    "serviceBus": {
      "maxConcurrentCalls": 16,
      "prefetchCount": 100,
      "autoRenewTimeout": "00:05:00"
    }
}
Property Default Description
maxConcurrentCalls 16 The maximum number of concurrent calls to the callback that the message pump should initiate. By default, the Functions runtime processes multiple messages concurrently. To direct the runtime to process only a single queue or topic message at a time, set maxConcurrentCalls to 1.
prefetchCount n/a The default PrefetchCount that will be used by the underlying MessageReceiver.
autoRenewTimeout 00:05:00 The maximum duration within which the message lock will be renewed automatically.

singleton

Configuration settings for Singleton lock behavior. For more information, see GitHub issue about singleton support.

{
    "singleton": {
      "lockPeriod": "00:00:15",
      "listenerLockPeriod": "00:01:00",
      "listenerLockRecoveryPollingInterval": "00:01:00",
      "lockAcquisitionTimeout": "00:01:00",
      "lockAcquisitionPollingInterval": "00:00:03"
    }
}
Property Default Description
lockPeriod 00:00:15 The period that function level locks are taken for. The locks auto-renew.
listenerLockPeriod 00:01:00 The period that listener locks are taken for.
listenerLockRecoveryPollingInterval 00:01:00 The time interval used for listener lock recovery if a listener lock couldn't be acquired on startup.
lockAcquisitionTimeout 00:01:00 The maximum amount of time the runtime will try to acquire a lock.
lockAcquisitionPollingInterval n/a The interval between lock acquisition attempts.

tracing

Configuration settings for logs that you create by using a TraceWriter object. See C# Logging and Node.js Logging.

{
    "tracing": {
      "consoleLevel": "verbose",
      "fileLoggingMode": "debugOnly"
    }
}
Property Default Description
consoleLevel info The tracing level for console logging. Options are: off, error, warning, info, and verbose.
fileLoggingMode debugOnly The tracing level for file logging. Options are never, always, debugOnly.

watchDirectories

A set of shared code directories that should be monitored for changes. Ensures that when code in these directories is changed, the changes are picked up by your functions.

{
    "watchDirectories": [ "Shared" ]
}

durableTask

Task hub name for Durable Functions.

{
  "durableTask": {
    "HubName": "MyTaskHub"
  }
}

Task hub names must start with a letter and consist of only letters and numbers. If not specified, the default task hub name for a function app is DurableFunctionsHub. For more information, see Task hubs.

Next steps