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await

Await availability of resources.

This can be useful in the context of Docker Compose where a service needs to wait for other dependent services.

Optionally a timeout can be provided to specify how long to wait for all dependent resources to become available. On success the command returns code 0, on failure it returns code 1.

Additionally a command can be specified which gets executed after all dependent resources became available.

Installation

go get -u github.com/djui/await

or

curl -s -f -L -o await https://github.com/djui/await/releases/download/v1.f2.0/await-linux-amd64
chmod +x await

Usage

$ await -h
Usage: await [options...] <res>... [ -- <cmd>]
Await availability of resources.

  -V	Show version
  -f	Force running the command even after giving up
  -i string
    	Read resources from file, '-' to read from stdin
  -q	Set quiet mode
  -t duration
    	Set timeout duration before giving up (default 1m0s)
  -v	Set verbose output mode
  -vv
    	Set more verbose output mode

Resources

All dependent resources must be specified as URLs or escaped command.

Some resources provided additional functionally encoded as fragment (#<fragment>). The syntax follows the URL query syntax: k1|k1=|k1=v1,v2,v3...[&k2=v1&...]. E.g.: http://example.com/#ssl&foo=bar,baz&i=j

Valid resources are: HTTP, Websocket, TCP, File, PostgreSQL, MySQL, Command.

HTTP Resource

Availability: Available when a connection to a given server is established and an empty request returns the response status code is 2xx. Unavailable otherwise.

URL syntax: http[s]://[<user>[:<pass>]@]<host>[:<port>][<path>][?<query>][#<fragment>]

Fragment:

  • tls=skip-verify: When used, it skips TLS check for https resources.

Websocket Resource

Availability: Available when a connection to a given server is established. Unavailable otherwise.

URL syntax: ws[s]://[<user>[:<pass>]@]<host>[:<port>][<path>][?<query>]

TCP Resource

Availability: Available when a connection to a given server is established. Unavailable otherwise.

URL syntax: tcp[4|6]://<host>[:<port>]

File Resource

Availability: Available when given path is a file and the file exists or, if declared absent, the file does not exist. Unavailable otherwise.

URL syntax: file://<path>[#<fragment>]

  • absent key: If present, the resource is defined as available, when the specific file is absent, rather than existing.

PostgreSQL Resource

Availability: Available when a connection to a given PostgreSQL database server is established and optional a given database could be found and optional any tables or a set of tables could be found. Unavailable otherwise.

URL syntax: postgres://[<user>[:<pass>]@]<host>[:<port>][/<dbname>][?<dbparams>][#<fragment>]

The URL defines a DSN.

The database name <dbname> is optional. If provided, the resource is classified as available as soon as the database was found.

DB Parameters:

  • sslmode=[verify-ca|require]: sslmode=verify-ca enables TLS/SSL encrypted connection to the server. Use sslmode=require if you want to use a self-signed or invalid certificate (server side). See lib/pq for more details.

Fragment:

  • tables[=t1,t2,...] key-value: If key present and value absent, the resource's database scheme must at least contain one table. If key present and value present, the resource's database scheme must at least contain the specified tables. Using this key requires to provide a database name.

MySQL Resource

Availability: Available when a connection to a given MySQL database server is established and optional a given database could be found and optional any tables or a set of tables could be found. Unavailable otherwise.

URL syntax: mysql://[<user>[:<pass>]@]<host>[:<port>][/<dbname>][?<dbparams>][#<fragment>]

The URL defines a DSN.

The database name <dbname> is optional. If provided, the resource is classified as available as soon as the database was found.

DB Parameters:

  • tls=[true|skip-verify]: tls=true enables TLS/SSL encrypted connection to the server. Use tls=skip-verify if you want to use a self-signed or invalid certificate (server side). See go-sql-driver/mysql for more details.

Fragment:

  • tables[=t1,t2,...] key-value: If key present and value absent, the resource's database scheme must at least contain one table. If key present and value present, the resource's database scheme must at least contain the specified tables. Using this key requires to provide a database name.

AMQP resource

Availability: Available when a connection to the given AMQP server is established. Unavailable otherwise.

URL syntax: amqp[s]://[<user>[:<pass>]@]<host>[:<port>]/[<vhost>]

Command Resource

Does not follow the URL syntax and is used as generic fallback for invalid URLs (i.e. schema is absent).

Availability: Available when command return status code 0. Unavailable otherwise.

URL syntax: <path> [<arg>...]

Alternatives

Once Docker-Compose support the Docker HEALTHCHECK directive for awaiting services being up, this tool could become deprecated.

Many alternative solutions exist, most of them avoiding to interpret the resource type, leaving only options like tcp and http, or having a specific focus on Docker Compose. A few of them listed below:

License

MIT License. See LICENSE.