Account lookup using Resolver

The resolver configuration option is used in conjunction with NATS JWT Authentication and nsc. The resolver option specifies a URL where the nats-server can retrieve an account JWT. There are 3 resolver implementations:
​NATS Based Resolver which is the preferred option and should be your default selection
​MEMORY if you want to statically define the accounts in the server configuration
​URL if you want to build your own account service, typically in order to have some integration of NATS security with some external security system.
If the operator JWT specified in operator contains an account resolver URL, resolver only needs to be specified in order to overwrite that default.
NATS Based Resolver
The NATS based resolver is the preferred and easiest way to enable account lookup for the nats servers. It is built-in into nats-server and stores the account JWTs in a local (not shared) directory that the server has access to (i.e. you can't have more than one nats-servers using the same directory. All the servers in the cluster or super-cluster must be configured to use it, and they implement an 'eventually consistent' mechanism via NATS and the system account to synchronize (or lookup) the account data between themselves.
In order to avoid having to store all account JWT on every nats-server (i.e. if you have a lot of accounts), this resolver has two sub types full and cache.
In this mode of operation administrators typically use the nsc CLI tool to create/manage the JWTs locally, and use nsc push to push new JWTs to the nats-servers' built-in resolvers, nsc pull to refresh their local copy of account JWTs, and nsc revocations to revoke them.
Full
The Full resolver means that the nats-server stores all JWTs and exchanges them in an eventually consistent way with other resolvers of the same type.
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resolver: {
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    type: full
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    # Directory in which account jwt will be stored
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    dir: './jwt'
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    # In order to support jwt deletion, set to true
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    # If the resolver type is full delete will rename the jwt.
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    # This is to allow manual restoration in case of inadvertent deletion.
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    # To restore a jwt, remove the added suffix .delete and restart or send a reload signal.
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    # To free up storage you must manually delete files with the suffix .delete.
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    allow_delete: false
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    # Interval at which a nats-server with a nats based account resolver will compare
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    # it's state with one random nats based account resolver in the cluster and if needed,
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    # exchange jwt and converge on the same set of jwt.
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    interval: "2m"
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    # limit on the number of jwt stored, will reject new jwt once limit is hit.
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    limit: 1000
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}
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This resolver type also supports resolver_preload. When present, JWTs are listed and stored in the resolver. There, they may be subject to updates. Restarts of the nats-server will hold on to these more recent versions.
Not every server in a cluster needs to be set to full. You need enough to still serve your workload adequately, while some servers are offline.
Cache
The Cache resolver means that the nats-server only stores a subset of the JWTs and evicts others based on an LRU scheme. Missing JWTs are downloaded from the full nats based resolver(s).
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resolver: {
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    type: cache
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    # Directory in which account jwt will be store
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    dir: "./"
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    # limit on the number of jwt stored, will evict old jwt once limit is hit.
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    limit: 1000
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    # How long to hold on to a jwt before discarding it. 
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    ttl: "2m"
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}
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NATS Based Resolver - Integration
The NATS based resolver utilizes the system account for lookup and upload of account JWTs. If your application requires tighter integration you can make use of these subjects for tighter integration.
To upload or update any generated account JWT without nsc, send it as a request to $SYS.REQ.CLAIMS.UPDATE. Each participating full NATS based account resolver will respond with a message detailing success or failure.
To serve a requested account JWT yourself and essentially implement an account server, subscribe to $SYS.REQ.ACCOUNT.*.CLAIMS.LOOKUP and respond with the account JWT corresponding to the requested account id (wildcard).
Migrating account data
To migrate account data when you change from using the standalone (REST) account server to the built-in NATS account resolver (or between NATS environments, or account servers) you can use nsc:
1.Run nsc pull to make sure you have a copy of all the account data in the server in your local machine
2.Reconfigure your servers to use the nats resolver instead of the URL resolver
3.Modify the 'account server URL' setting in your operator to the nats URL from the old REST URL: i.e. just copy the nats URLs from the operator's 'service URLs' setting into the account server URLs. nsc edit operator --account-jwt-server-url <nats://...>
4.do nsc push -A to push your account data to the nats-servers using the built-in nats account resolver
You can also pass the account server URLs directly as a flag to the nsc pull and nsc push commands.
MEMORY
The MEMORY resolver is statically configured in the server's configuration file. You would use this mode if you would rather manage the account resolving 'by hand' through the nat-servers' configuration files. The memory resolver makes use of the resolver_preload directive, which specifies a map of public keys to account JWTs:
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resolver: MEMORY
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resolver_preload: {
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ACSU3Q6LTLBVLGAQUONAGXJHVNWGSKKAUA7IY5TB4Z7PLEKSR5O6JTGR: eyJ0eXAiOiJqd3QiLCJhbGciOiJlZDI1NTE5In0.eyJqdGkiOiJPRFhJSVI2Wlg1Q1AzMlFJTFczWFBENEtTSDYzUFNNSEZHUkpaT05DR1RLVVBISlRLQ0JBIiwiaWF0IjoxNTU2NjU1Njk0LCJpc3MiOiJPRFdaSjJLQVBGNzZXT1dNUENKRjZCWTRRSVBMVFVJWTRKSUJMVTRLM1lERzNHSElXQlZXQkhVWiIsIm5hbWUiOiJBIiwic3ViIjoiQUNTVTNRNkxUTEJWTEdBUVVPTkFHWEpIVk5XR1NLS0FVQTdJWTVUQjRaN1BMRUtTUjVPNkpUR1IiLCJ0eXBlIjoiYWNjb3VudCIsIm5hdHMiOnsibGltaXRzIjp7InN1YnMiOi0xLCJjb25uIjotMSwibGVhZiI6LTEsImltcG9ydHMiOi0xLCJleHBvcnRzIjotMSwiZGF0YSI6LTEsInBheWxvYWQiOi0xLCJ3aWxkY2FyZHMiOnRydWV9fX0._WW5C1triCh8a4jhyBxEZZP8RJ17pINS8qLzz-01o6zbz1uZfTOJGvwSTS6Yv2_849B9iUXSd-8kp1iMXHdoBA
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}
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The MEMORY resolver is recommended when the server has a small number of accounts that don't change very often.
For more information on how to configure a memory resolver, see this tutorial.
URL Resolver
NOTE: The standalone NATS Account JWT Server is now legacy, please use the NATS Based Resolver instead. However, the URL resolver option is still available in case you want to implement your own version of an account resolver
The URL resolver specifies a URL where the server can append an account public key to retrieve that account's JWT. Convention for standalone NATS Account JWT Servers is to serve JWTs at: http://localhost:9090/jwt/v1/accounts/. For such a configuration you would specify the resolver as follows:
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resolver: URL(http://localhost:9090/jwt/v1/accounts/)
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Note that if you are not using a nats-account-server, the URL can be anything as long as by appending the public key for an account, the requested JWT is returned.
If the server used requires client authentication, or you want to specify which CA is trusted for the lookup of account information, specify resolver_tls. This tls configuration map lets you further restrict TLS to the resolver.
Decentralized JWT Authentication/Authorization
Memory Resolver Tutorial
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Contents
NATS Based Resolver
Full
Cache
NATS Based Resolver - Integration
Migrating account data
MEMORY
URL Resolver