Horizon is responsible for providing an HTTP API to data in the Stellar network. It ingests and re-serves the data produced by the stellar network in a form that is easier to consume than the performance-oriented data representations used by stellar-core.
Why run horizon?
The stellar development foundation runs two horizon servers, one for the public network and one for the test network, free for anyone’s use at https://horizon.stellar.org and https://horizon-testnet.stellar.org. These servers should be fine for development and small scale projects, but is not recommended that you use them for production services that need strong reliability. By running horizon within your own infrastructure provides a number of benefits:
- Multiple instances can be run for redundancy and scalability.
- Request rate limiting can be disabled.
- Full operational control without dependency on the Stellar Development Foundations operations.
Horizon is dependent upon a stellar-core server. Horizon needs access to both the SQL database and the HTTP API that is published by stellar-core. See the administration guide to learn how to set up and administer a stellar-core server. Secondly, horizon is dependent upon a postgres server, which it uses to store processed core data for ease of use. Horizon requires postgres version >= 9.3.
In addition to the two prerequisites above, you may optionally install a redis server to be used for rate limiting requests.
To install horizon, you have a choice: either downloading a prebuilt release for your target architecture and operation system, or building horizon yourself. When either approach is complete, you will find yourself with a directory containing a file named
horizon. This file is a native binary.
After building or unpacking horizon, you simply need to copy the native binary into a directory that is part of your PATH. Most unix-like systems have
/usr/local/bin in PATH by default, so unless you have a preference or know better, we recommend you copy the binary there.
To test the installation, simply run
horizon --help from a terminal. If the help for horizon is displayed, your installation was successful. Note: some shells, such as zsh, cache PATH lookups. You may need to clear your cache (by using
rehash in zsh, for example) before trying to run
Should you decide not to use one of our prebuilt releases, you may instead build horizon from source. To do so, you need to install some developer tools:
- A unix-like operating system with the common core commands (cp, tar, mkdir, bash, etc.)
- A compatible distribution of go (we officially support go 1.6 and later)
Provided your workstation satisfies the requirements above, follow the steps below:
- Clone horizon’s source:
go get github.com/stellar/go && cd $GOPATH/src/github.com/stellar/go/
- Under the project folder, download external dependencies:
- Build the binary:
go install github.com/stellar/go/services/horizon
After running the above commands have succeeded, the built horizon will have been written into the
bin subdirectory of your $GOPATH.
Note: Building directly on windows is not supported.
Horizon is configured using command line flags or environment variables. To see the list of command line flags that are available (and their default values) for your version of horizon, run:
As you will see if you run the command above, horizon defines a large number of flags, however only three are required:
--db-url specifies the horizon database, and its value should be a valid PostgreSQL Connection URI.
--stellar-core-db-url specifies a stellar-core database which will be used to load data about the stellar ledger. Finally,
--stellar-core-url specifies the HTTP control port for an instance of stellar-core. This URL should be associated with the stellar-core that is writing to the database at
Specifying command line flags every time you invoke horizon can be cumbersome, and so we recommend using environment variables. There are many tools you can use to manage environment variables: we recommend either direnv or dotenv. A template configuration that is compatible with dotenv can be found in the horizon git repo.
Preparing the database
Before the horizon server can be run, we must first prepare the horizon database. This database will be used for all of the information produced by horizon, notably historical information about successful transactions that have occurred on the stellar network.
To prepare a database for horizon’s use, first you must ensure the database is blank. It’s easiest to simply create a new database on your postgres server specifically for horizon’s use. Next you must install the schema by running
horizon db init. Remember to use the appropriate command line flags or environment variables to configure horizon as explained in Configuring . This command will log any errors that occur.
Once your horizon database is configured, you’re ready to run horizon. To run horizon you simply run
horizon serve, both of which start the HTTP server and start logging to standard out. When run, you should see some output that similar to:
INFO Starting horizon on :8000 pid=29013
The log line above announces that horizon is ready to serve client requests. Note: the numbers shown above may be different for your installation. Next we can confirm that horizon is responding correctly by loading the root resource. In the example above, that URL would be [http://127.0.0.1:8000/] and simply running
Ingesting stellar-core data
curl http://127.0.0.1:8000/ shows you that the root resource can be loaded correctly.
Horizon provides most of its utility through ingested data. Your horizon server can be configured to listen for and ingest transaction results from the connected stellar-core. We recommend that within your infrastructure you run one (and only one) horizon process that is configured in this way. While running multiple ingestion processes will not corrupt the horizon database, your error logs will quickly fill up as the two instances race to ingest the data from stellar-core. We may develop a system that coordinates multiple horizon processes in the future, but we would also be happy to include an external contribution that accomplishes this.
To enable ingestion, you must either pass
Managing storage for historical data
--ingest=true on the command line or set the
INGEST environment variable to “true”.
Given an empty horizon database, any and all available history on the attached stellar-core instance will be ingested. Over time, this recorded history will grow unbounded, increasing storage used by the database. To keep your costs down, you may configure horizon to only retain a certain number of ledgers in the historical database. This is done using the
Surviving stellar-core downtime
--history-retention-count flag or the
HISTORY_RETENTION_COUNT environment variable. Set the value to the number of recent ledgers you wish to keep around, and every hour the horizon subsystem will reap expired data. Alternatively, you may execute the command
horizon db reap to force a collection.
Horizon tries to maintain a gap-free window into the history of the stellar-network. This reduces the number of edge cases that horizon-dependent software must deal with, aiming to make the integration process simpler. To maintain a gap-free history, horizon needs access to all of the metadata produced by stellar-core in the process of closing a ledger, and there are instances when this metadata can be lost. Usually, this loss of metadata occurs because the stellar-core node went offline and performed a catchup operation when restarted.
To ensure that the metadata required by horizon is maintained, you have several options: You may either set the
CATCHUP_COMPLETE stellar-core configuration option to
true or configure
CATCHUP_RECENT to determine the amount of time your stellar-core can be offline without having to rebuild your horizon database.
We do not recommend using the
Correcting gaps in historical data
CATCHUP_COMPLETE method, as this will force stellar-core to apply every transaction from the beginning of the ledger, which will take an ever increasing amount of time. Instead, we recommend you set the
CATCHUP_RECENT config value. To do this, determine how long of a downtime you would like to survive (expressed in seconds) and divide by ten. This roughly equates to the number of ledgers that occur within your desired grace period (ledgers roughly close at a rate of one every ten seconds). With this value set, stellar-core will replay transactions for ledgers that are recent enough, ensuring that the metadata needed by horizon is present.
In the section above, we mentioned that horizon tries to maintain a gap-free window. Unfortunately, it cannot directly control the state of stellar-core and so gaps may form due to extended down time. When a gap is encountered, horizon will stop ingesting historical data and complain loudly in the log with error messages (log lines will include “ledger gap detected”). To resolve this situation, you must re-establish the expected state of the stellar-core database and purge historical data from horizon’s database. We leave the details of this process up to the reader as it is dependent upon your operating needs and configuration, but we offer one potential solution:
We recommend you configure the HISTORY_RETENTION_COUNT in horizon to a value less than or equal to the configured value for CATCHUP_RECENT in stellar-core. Given this situation any downtime that would cause a ledger gap will require a downtime greater than the amount of historical data retained by horizon. To re-establish continuity, simply:
Managing Stale Historical Data
- Stop horizon.
horizon db reap to clear the historical database.
- Clear the cursor for horizon by running
stellar-core -c "dropcursor?id=HORIZON" (ensure capitilization is maintained).
- Clear ledger metadata from before the gap by running
stellar-core -c "maintenance?queue=true".
- Restart horizon.
Horizon ingests ledger data from a connected instance of stellar-core. In the event that stellar-core stops running (or if horizon stops ingesting data for any other reason), the view provided by horizon will start to lag behind reality. For simpler applications, this may be fine, but in many cases this lag is unacceptable and the application should not continue operating until the lag is resolved.
To help applications that cannot tolerate lag, horizon provides a configurable “staleness” threshold. Given that enough lag has accumulated to surpass this threshold (expressed in number of ledgers), horizon will only respond with an error:
stale_history. To configure this option, use either the
--history-stale-threshold command line flag or the
HISTORY_STALE_THRESHOLD environment variable. NOTE: non-historical requests (such as submitting transactions or finding payment paths) will not error out when the staleness threshold is surpassed.
To ensure that your instance of horizon is performing correctly we encourage you to monitor it, and provide both logs and metrics to do so.
Horizon will output logs to standard out. Information about what requests are coming in will be reported, but more importantly, warnings or errors will also be emitted by default. A correctly running horizon instance will not output any warning or error log entries.
Metrics are collected while a horizon process is running and they are exposed at the
I’m Stuck! Help!
/metrics path. You can see an example at (https://horizon-testnet.stellar.org/metrics).
If any of the above steps don’t work or you are otherwise prevented from correctly setting up horizon, please come to our community and tell us. Either post a question at our Stack Exchange or chat with us on slack to ask for help.
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