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remove dup info, improve clarity, add samples
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@trevorbye
trevorbye committed Nov 25, 2021
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* [ES6 Tagged template literals](#es6-tagged-template-literals)
* [Callbacks](#callbacks)
* [Streaming](#streaming)
* [Connection Pools](#connection-pools)
* [Connection Pools](#advanced-pool-management)

### Configuration

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## Pool Management

An important concept to understand when using this library is [Connection Pooling](https://en.wikipedia.org/wiki/Connection_pool)
as this library uses connection pooling extensively.

As one Node JS process is able to handle multiple requests at once, we can take advantage of this long running process
to create a pool of database connections for reuse; this saves overhead of connecting to the database for each request
An important concept to understand when using this library is [Connection Pooling](https://en.wikipedia.org/wiki/Connection_pool) as this library uses connection pooling extensively. As one Node JS process is able to handle multiple requests at once, we can take advantage of this long running process to create a pool of database connections for reuse; this saves overhead of connecting to the database for each request
(as would be the case in something like PHP, where one process handles one request).

With the advantages of pooling comes some added complexities, but these are mostly just conceptual and once you understand
how the pooling is working, it is simple to make use of it efficiently and effectively.
With the advantages of pooling comes some added complexities, but these are mostly just conceptual and once you understand how the pooling is working, it is simple to make use of it efficiently and effectively.

### The Global Connection (Pool)
### The Global Connection Pool

To assist with pool management in your application there is the global `connect()` function that is available for use. As of
v6 of this library a developer can make repeated calls to this function to obtain the global connection pool. This means you
do not need to keep track of the pool in your application (as used to be the case). If the global pool is already connected,
it will resolve to the connected pool. For example:
To assist with pool management in your application there is the `sql.connect()` function that you use to connect to the global connection pool. You can make repeated calls to this function, and if the global pool is already connected, it will resolve to the connected pool. The following example obtains the global connection pool by running `sql.connect()`, and then runs the query against the pool.

```js
const sql = require('mssql')
Expand All @@ -399,235 +391,177 @@ function runQuery(query) {
return pool.query(query)
})
}

```

Here we obtain the global connection pool by running `sql.connect()` and we then run the query against the pool.
We also do *not* close the pool after the query is executed and that is because other queries may need to be run against
this pool and closing it will add an overhead to running the query. We should only ever close the pool when our application
is finished. For example, if we are running some kind of CLI tool or a CRON job:
Awaiting or `.then`-ing the pool creation is a safe way to ensure that the pool is always ready, without knowing where it is needed first. In practice, once the pool is created then there will be no delay for the next `connect()` call.

```js
const sql = require('mssql')
Also notice that we do *not* close the global pool by calling `sql.close()` after the query is executed, because other queries may need to be run against this pool and closing it will add additional overhead to running subsequent queries. You should only ever close the global pool if you're certain the application is finished. Or for example, if you are running some kind of CLI tool or a CRON job you can close the pool at the end of the script.

(() => {
sql.connect().then(pool => {
return pool.query('SELECT 1')
}).then(result => {
// do something with result
}).then(() => {
return sql.close()
})
})()
```
### Global Pool Single Instance

Here the connection will be closed and the node process will exit once the queries and other application logic has completed.
You should aim to only close the pool once in your application, when it is exiting or you know your application will never make
another SQL query.
The ability to call `connect()` and `close()` repeatedly on the global pool is intended to make pool management easier, however it is generally recommended to follow a singleton design when possible, where `connect()` is called **once**, and the resulting global pool connection promise is re-used throughout the entire application. Repeatedly calling `connect()` can cause connection issues (e.g. `ECONNCLOSED`) or other anomalies in some circumstances like Express applications or Azure Functions/Azure SQL Server, depending on:

### Advanced Pool Management

In some instances you will not want to use the connection pool, you may have multiple databases to connect to or you may have
one pool for read-only operations and another pool for read-write. In this instance you will need to implement your own pool
management.
* Your specific implementation
* Usage with other libraries
* The databases (and their versions) that you are connecting to

That could look something like this:
In use-cases where you are able to maintain a single instance of a class to manage your database connectivity, use the following example as a guide. This class creates an instance variable to store a reference to the global connection pool, and queries reference this instance variable for fetching the global connection pool.

```js
const { ConnectionPool } = require('mssql')
const POOLS = {}

function createPool(config, name) {
if (getPool(name)) {
return Promise.reject(new Error('Pool with this name already exists'))
}
return (new ConnectionPool(config)).connect().then((pool) => {
return POOLS[name] = pool
})
}
const sql = require('mssql');

function closePool(name) {
const pool = getPool(name)
if (pool) {
delete POOLS[name]
return pool.close()
}
return Promise.resolve()
}

function getPool(name) {
if (Object.prototype.hasOwnProperty.apply(POOLS, name)) {
return POOLS[name]
}
}

module.exports = {
closePool,
createPool,
getPool
}
```

This helper file can then be used in your application to create, fetch and close your pools. As with the global pools, you
should aim to only close a pool when you know it will never be needed by the application again; typically this will be when
your application is shutting down.

## Connection Pools

Using a single connection pool for your application/service is recommended.
Instantiating a pool with a callback, or immediately calling `.connect`, is asynchronous to ensure a connection can be
established before returning. From that point, you're able to acquire connections as normal:

```javascript
const sql = require('mssql')

// async/await style:
const pool1 = new sql.ConnectionPool(config);
const pool1Connect = pool1.connect();
class CustomConnector {
constructor(config) {
this.globalPool = null;
this.config = config;
}

pool1.on('error', err => {
// ... error handler
})
// call immediately after constructor to create singleton global pool
async init() {
this.globalPool = await sql.connect(this.config);
}

async function messageHandler() {
await pool1Connect; // ensures that the pool has been created
try {
const request = pool1.request(); // or: new sql.Request(pool1)
const result = await request.query('select 1 as number')
console.dir(result)
async runQuery(query) {
// references previously connected global pool from instance variable
const result = await this.globalPool.request().query(query);
return result;
} catch (err) {
console.error('SQL error', err);
}
}

// promise style:
const pool2 = new sql.ConnectionPool(config)
const pool2Connect = pool2.connect()

pool2.on('error', err => {
// ... error handler
})

function runStoredProcedure() {
return pool2Connect.then((pool) => {
pool.request() // or: new sql.Request(pool2)
.input('input_parameter', sql.Int, 10)
.output('output_parameter', sql.VarChar(50))
.execute('procedure_name', (err, result) => {
// ... error checks
console.dir(result)
})
}).catch(err => {
// ... error handler
})
async function startApp() {
const config = {
user: '...',
password: '...',
server: 'localhost',
database: '...',
pool: {
max: 10,
min: 0,
idleTimeoutMillis: 30000
}
}
// customConnector is the only instance of this class used in the application
const customConnector = new CustomConnector(config);
await customConnector.init();
const queryResult = await customConnector.runQuery('SELECT TOP 10 * FROM table_name');
}
```

Awaiting or `.then`ing the pool creation is a safe way to ensure that the pool is always ready, without knowing where it
is needed first. In practice, once the pool is created then there will be no delay for the next operation.

As of v6.1.0 you can make repeat calls to `ConnectionPool.connect()` and `ConnectionPool.close()` without an error being
thrown, allowing for the safe use of `mssql.connect().then(...)` throughout your code as well as making multiple calls to
close when your application is shutting down.

The ability to call `connect()` repeatedly is intended to make pool management easier, however it is still recommended
to follow the example above where `connect()` is called once and using the original resolved connection promise.
Repeatedly calling `connect()` when running queries risks running into problems when `close()` is called on the pool.
To extend this example and make it more scaleable, you could include a check for failed connections and then call `sql.connect()` again to refresh the `globalPool` instance variable.

**ES6 Tagged template literals**
For Express applications, the following example uses a single global pool instance constant that is passed into the routes. The server start is then chained inside the `connect()` promise.

```javascript
new sql.ConnectionPool(config).connect().then(pool => {
return pool.query`select * from mytable where id = ${value}`
}).then(result => {
console.dir(result)
}).catch(err => {
// ... error checks
})
```js
const express = require('express');
const sql = require('mssql');
const config = {/*...*/};
//instantiate a connection pool
const globalPool = new sql.Connection(config);
//require route handlers and use the same connection pool everywhere
const route1 = require('./routes/route1')(globalPool);
const app = express();
app.get('/path', route1.get);

//connect the pool and start the web server when done
globalPool.connect().then(function() {
const server = app.listen(3000, function () {
const host = server.address().address;
const port = server.address().port;
console.log('Example app listening at http://%s:%s', host, port);
});
}).catch(function(err) {
console.error('Error creating connection pool', err);
});
```

All values are automatically sanitized against sql injection.

### Managing connection pools

Most applications will only need a single `ConnectionPool` that can be shared throughout the code. To aid the sharing
of a single pool this library exposes a set of functions to access a single global connection. eg:
Then the route uses the passed global connection pool object.

```js
// as part of your application's boot process
// ./routes/route1.js
const sql = require('mssql');

module.exports = function(globalPool) {
const me = {
get: function(req, res, next) {
const request = new sql.Request(globalPool);
request.query('SELECT TOP 10 * FROM table_name', function(err, recordset) {
if (err) {
console.error(err);
res.status(500).send(err.message);
return;
}
res.status(200).json(recordset);
});
}
};

const sql = require('mssql')
const poolPromise = sql.connect()
return me;
};
```

// during your applications runtime
### Advanced Pool Management

poolPromise.then(() => {
return sql.query('SELECT 1')
}).then(result => {
console.dir(result)
})
For some use-cases you may want to implement your own connection pool management, rather than using the global connection pool. Reasons for doing this include:

// when your application exits
poolPromise.then(() => {
return sql.close()
})
```
* Supporting connections to multiple databases
* Creation of separate pools for read vs read/write operations

If you require multiple pools per application (perhaps you have many DBs you need to connect to or you want a read-only
pool), then you will need to manage your pools yourself. The best way to do this is to create a shared library file that
can hold references to the pools for you. For example:
The following code is an example of a custom connection pool implementation.

```js
const sql = require('mssql')
// CustomPoolManager.js
const { ConnectionPool } = require('mssql')
const POOLS = {}

const pools = {}

// manage a set of pools by name (config will be required to create the pool)
// a pool will be removed when it is closed
async function getPool(name, config) {
if (!Object.prototype.hasOwnProperty.call(pools, name)) {
const pool = new sql.ConnectionPool(config)
const close = pool.close.bind(pool)
pool.close = (...args) => {
delete pools[name]
return close(...args)
}
await pool.connect()
pools[name] = pool
async function createPool(config, name) {
if (getPool(name)) {
return Promise.reject(new Error('Pool with this name already exists'))
}
return pools[name]
const pool = new ConnectionPool(config)
await pool.connect()
return POOLS[name] = pool
}

// close all pools
function closeAll() {
return Promise.all(Object.values(pools).map((pool) => {
return pool.close()
}))
async function closePool(name) {
const pool = getPool(name)
if (pool) {
delete POOLS[name]
await pool.close()
}
}

function getPool(name) {
if (Object.prototype.hasOwnProperty.apply(POOLS, name)) {
return POOLS[name]
}
}

module.exports = {
closeAll,
closePool,
createPool,
getPool
}
```

You can then use this library file in your code to get a connected pool when you need it:
This file can then be used in your application to create, fetch, and close pools.

```js
const { getPool } = require('./path/to/file')
const { getPool } = require('./CustomPoolManager.js')

// run a query
async function runQuery(query, config) {
async function runQuery(query) {
// pool will always be connected when the promise has resolved - may reject if the connection config is invalid
const pool = await getPool('default', config)
const pool = getPool('default')
const result = await pool.request().query(query)
return result
}
```

Similar to the global connection pool, you should aim to only close a pool when you know it will never be needed by the application again. Typically this will only be when your application is shutting down.

## Configuration

The following is an example configuration object:

```javascript
const config = {
user: '...',
Expand Down Expand Up @@ -1931,7 +1865,7 @@ Name | Code | Message
`ConnectionError` | ENOTOPEN | Connection not yet open.
`ConnectionError` | EINSTLOOKUP | Instance lookup failed.
`ConnectionError` | ESOCKET | Socket error.
`ConnectionError` | ECONNCLOSED | Connection is closed.
`ConnectionError` | ECONNCLOSED | Connection is closed. See [global pool single instance](#global-pool-single-instance).
`TransactionError` | ENOTBEGUN | Transaction has not begun.
`TransactionError` | EALREADYBEGUN | Transaction has already begun.
`TransactionError` | EREQINPROG | Can't commit/rollback transaction. There is a request in progress.
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