Server Methods in hapi

Overview

Server methods are a useful way of sharing functions by attaching them to your server object rather than requiring a common module everywhere it is needed. Server methods are also used heavily for caching purposes. Since server methods leverage hapi's native caching, they can help reduce your boilerplate to a minimum. See the caching tutorial for more. To register a server method, you call server.method(). There are two different ways to call this function. You can call it with the signature server.method(name, method, [options]), or you can call it with the signature server.method(method), where method is an object with name, method, and options parameters (note that you may also pass an array of these objects).

server.method()

The first way to call server.method() is with the signature server.method(name, method, [options]):

const add = function (x, y) {

    return x + y;
};

server.method('add', add, {});

Here, you create a function called add, which takes two parameters and adds them together. Then you call server.method() with the name of the method being add, the method you are using, the one we just created called add, and no options.

The second way to call server.method() is with the signature server.method(method):

const add = function (x, y) {

    return x + y;
};

server.method({
    name: 'add',
    method: add,
    options: {}
});

You create the same function again, called add. When you register it this time, configure the methodobject. This case, name is the name of the method, method is the method you are using, and optionsis an object to configure various options.

Name

The name parameter is a string used to retrieve the method from the server later, via server.methods[name]. Note that if you specify a name with a . character, it is registered as a nested object rather than the literal string. As in:

server.method('math.add', add);

This server method can then be called via server.methods.math.add().

Method

The method parameter is the actual function to call when the method is invoked. It can take any number of arguments. It can be an async function, for example:

const add = async function (x, y) {

    const result = await someLongRunningFunction(x, y);
    return result;
};

server.method('add', add, {});

Your server method function should return a valid result or throw an error if one occurs.

Options

When registering server.method(), you can configure three options: cache, generateKey, and bind.

Cache

A major advantage of server methods is that they may leverage hapi's native caching. The default is to not cache, however if a valid configuration is passed when registering the method, the return value will be cached and served from the cache instead of re-running your method every time it is called. The configuration looks like the following:

server.method('add', add, {
    cache: {
        expiresIn: 60000,
        expiresAt: '20:30',
        staleIn: 30000,
        staleTimeout: 10000,
        generateTimeout: 100
    }
});

The parameters mean:

• expiresIn: relative expiration expressed in the number of milliseconds since the item was saved in the cache. Cannot be used together with expiresAt.
• expiresAt: time of day expressed in 24h notation using the 'HH:MM' format, at which point all cache records for the route expire. Uses local time. Cannot be used together with expiresIn.
• staleIn: number of milliseconds to mark an item stored in cache as stale and attempt to regenerate it. Must be less than expiresIn.
• staleTimeout: number of milliseconds to wait before returning a stale value while generateFunc is generating a fresh value.
• generateTimeout: number of milliseconds to wait before returning a timeout error when it takes too long to return a value. When the value is eventually returned, it is stored in the cache for future requests.
• segment: an optional segment name used to isolate cache items.
• cache: an optional string with the name of the cache connection configured on your server to use

More information on the caching options can be found in the API Reference as well as the documentation for catbox.

You can override the ttl (time-to-live) of a server method result per-invocation by setting the ttlflag. Let's see how this works with the earlier example:

const add = async function (x, y, flags) {

    const result = await someLongRunningFunction(x, y);

    flags.ttl = 5 * 60 * 1000; // 5 mins

    return result;
};

server.method('add', add, {
    cache: {
        expiresIn: 2000,
        generateTimeout: 100
    }
});

Here you defined your server method function to have one more parameter than you're expecting to pass to it, the additional flags parameter is passed by hapi. You then simply set the ttl flag to however long you want the result to be cached for (in milliseconds); if it is set to 0 then the value will never be cached. If you set no flag then the ttl will be taken from the cache configuration.

Generate a Custom Key

In addition to the above options, you may also define a custom function used to generate a cache key based on the parameters passed to your method. If your method only accepts some combination of string, number, and boolean values hapi will generate a sane key for you. However, if your method accepts an object parameter, you should specify a function that will generate a cache key similar to the following:

const sum = function (array) {

    let total = 0;

    array.forEach((item) => {

        total += item;
    });

    return total;
};

server.method('sum', sum, {
    generateKey: (array) => array.join(',')
});

Any arguments that you pass to your method are available to the generateKey method.

Bind

The last option available to server methods is bind. The bind option changes the this context within the method. It defaults to the current active context when the method is added. This can be useful for passing in a database client without needing to pass it as a parameter and requiring a custom generateKey function, as in:

const lookup = async function (id) {

    // calls myDB.getOne

    return await this.getOne({ id });
};

server.method('lookup', lookup, { bind: myDB });

server.methods

To call the server methods we registered above, you would use server.methods(). Consider our add function:

const add = function (x, y) {

    return x + y;
};

server.method({
    name: 'add',
    method: add,
    options: {}
});

To use this method, simply call server.methods()

server.methods.add(1, 2);  // 3