libpq

node-libpq

Node native bindings to the PostgreSQL libpq C client library. This module attempts to mirror as closely as possible the C API provided by libpq and provides the absolute minimum level of abstraction. It is intended to be extremely low level and allow you the same access as you would have to libpq directly from C, except in node.js! The obvious trade-off for being "close to the metal" is having to use a very "c style" API in JavaScript.

If you have a good understanding of libpq or used it before hopefully the methods within node-libpq will be familiar; otherwise, you should probably spend some time reading the official libpq C library documentation to become a bit familiar. Referencing the libpq documentation directly should also provide you with more insight into the methods here. I will do my best to explain any differences from the C code for each method.

I am also building some higher level abstractions to eventually replace the pg.native portion of node-postgres. They should help as reference material.

install

You need libpq installed & the pg_config program should be in your path. You also need node-gyp installed.

$ npm install libpq

use

var Libpq = require('libpq');
var pq = new Libpq();

API

connection functions

Libpq provides a few different connection functions, some of which are "not preferred" anymore. I've opted to simplify this interface a bit into a single async and single sync connnection function. The function accepts an connection string formatted as outlined in this documentation in section 31.1.1. If the parameters are not supplied, libpq will automatically use environment variables, a pgpass file, and other options. Consult the libpq documentation for a better rundown of all the ways it tries to determine your connection parameters.

I personally always connect with environment variables and skip supplying the optional connectionParams. Easier, more 12 factor app-ish, and you never risk hard coding any passwords. YMMV. :smile:

pq.connect([connectionParams:string], callback:function)

Asyncronously attempts to connect to the postgres server.

• connectionParams is an optional string
• callback is mandatory. It is called when the connection has successfully been established.

async Connects to a PostgreSQL backend server process.

This function actually calls the PQconnectdb blocking connection method in a background thread within node's internal thread-pool. There is a way to do non-blocking network I/O for some of the connecting with libpq directly, but it still blocks when your local file system looking for config files, SSL certificates, .pgpass file, and doing possible dns resolution. Because of this, the best way to get fully non-blocking is to juse use libuv_queue_work and let node do it's magic and so that's what I do. This function does not block.

pq.connectSync([connectionParams:string])

Attempts to connect to a PostgreSQL server. BLOCKS until it either succeedes, or fails. If it fails it will throw an exception.

• connectionParams is an optional string

pq.finish()

Disconnects from the backend and cleans up all memory used by the libpq connection.

Connection Status Functions

pq.errorMessage():string

Retrieves the last error message from the connection. This is intended to be used after most functions which return an error code to get more detailed error information about the connection. You can also check this before issuing queries to see if your connection has been lost.

pq.socket():int

Returns an int representing the file descriptor for the socket used internally by the connection

Sync Command Execution Functions

pq.exec(commandText:string)

sync sends a command to the backend and blocks until a result is received.

• commandText is a required string of the query.

pq.execParams(commandText:string, parameters:array[string])

snyc sends a command and parameters to the backend and blocks until a result is received.

• commandText is a required string of the query.
• parameters is a required array of string values corresponding to each parameter in the commandText.

pq.prepare(statementName:string, commandText:string, nParams:int)

sync sends a named statement to the server to be prepared for later execution. blocks until a result from the prepare operation is received.

• statementName is a required string of name of the statement to prepare.
• commandText is a required string of the query.
• nParams is a count of the number of parameters in the commandText.

pq.execPrepared(statementName:string, parameters:array[string])

sync sends a command to the server to execute a previously prepared statement. blocks until the results are returned.

• statementName is a required string of the name of the prepared statement.
• parameters are the parameters to pass to the prepared statement.

Async Command Execution Functions

In libpq the async command execution functions only dispatch a request to the backend to run a query. They do not start result fetching on their own. Because libpq is a C api there is a somewhat complicated "dance" to retrieve the result information in a non-blocking way. node-libpq attempts to do as little as possible to abstract over this; therefore, the following functions are only part of the story. For a complete tutorial on how to dispatch & retrieve results from libpq in an async way you can view the complete approach here

pq.sendQuery(commandText:string):boolean

async sends a query to the server to be processed.

• commandText is a required string containing the query text.

Returns true if the command was sent succesfully or false if it failed to send.

pq.sendQueryParams(commandText:string, parameters:array[string]):boolean

async sends a query and to the server to be processed.

• commandText is a required string containing the query text.
• parameters is an array of parameters as strings used in the parameterized query.

Returns true if the command was sent succesfully or false if it failed to send.

pq.sendPrepare(statementName:string, commandText:string, nParams:int):boolean

async sends a request to the backend to prepare a named statement with the given name.

• statementName is a required string of name of the statement to prepare.
• commandText is a required string of the query.
• nParams is a count of the number of parameters in the commandText.

Returns true if the command was sent succesfully or false if it failed to send.

pq.sendQueryPrepared(statementName:string, parameters:array[string]):boolean

async sends a request to execute a previously prepared statement.

• statementName is a required string of the name of the prepared statement.
• parameters are the parameters to pass to the prepared statement.

pq.getResult():boolean

Parses received data from the server into a PGresult struct and sets a pointer internally to the connection object to this result. warning: this function will block if libpq is waiting on async results to be returned from the server. Call pq.isBusy() to determine if this command will block.

Returns true if libpq was able to read buffered data & parse a result object. Returns false if there are no results waiting to be parsed. Generally doing async style queries you'll call this repeadedly until it returns false and then use the result accessor methods to pull results out of the current result set.

Result accessor functions

After a command is run in either sync or async mode & the results have been received, node-libpq stores the results internally and provides you access to the results via the standard libpq methods. The difference here is libpq will return a pointer to a PGresult structure which you access via libpq functions, but node-libpq stores the most recent result within itself and passes the opaque PGresult structure to the libpq methods. This is to avoid passing around a whole bunch of pointers to unmanaged memory and keeps the burden of properly allocating and freeing memory within node-libpq.

pq.resultStatus():string

Returns either PGRES_COMMAND_OK or PGRES_FATAL_ERROR depending on the status of the last executed command.

pq.resultErrorMessage():string

Retrieves the error message from the result. This will return null if the result does not have an error.

pq.resultErrorFields():object

Retrieves detailed error information from the current result object. Very similar to PQresultErrorField() except instead of passing a fieldCode and retrieving a single field, retrieves all fields from the error at once on a single object. The object returned is a simple hash, not an instance of an error object. Example: if you wanted to access PG_DIAG_MESSAGE_DETAIL you would do the following:

console.log(pq.errorFields().messageDetail)

pq.clear()

Manually frees the memory associated with a PGresult pointer. Generally this is called for you, but if you absolutely want to free the pointer yourself, you can.

pq.ntuples():int

Retrieve the number of tuples (rows) from the result.

pq.nfields():int

Retrieve the number of fields (columns) from the result.

pq.fname(fieldNumber:int):string

Retrieve the name of the field (column) at the given offset. Offset starts at 0.

pq.ftype(fieldNumber:int):int

Retrieve the Oid of the field (column) at the given offset. Offset starts at 0.

pq.getvalue(tupleNumber:int, fieldNumber:int):string

Retrieve the text value at a given tuple (row) and field (column) offset. Both offsets start at 0. A null value is returned as the empty string ''.

pq.getisnull(tupleNumber:int, fieldNumber:int):boolean

Returns true if the value at the given offsets is actually null. Otherwise returns false. This is because pq.getvalue() returns an empty string for both an actual empty string and for a null value. Weird, huh?

pq.cmdStatus():string

Returns the status string associated with a result. Something akin to INSERT 3 0 if you inserted 3 rows.

pq.cmdTuples():string

Returns the number of tuples (rows) affected by the command. Even though this is a number, it is returned as a string to mirror libpq's behavior.

Async socket access

These functions don't have a direct match within libpq. They exist to allow you to monitor the readability or writability of the libpq socket based on your platforms equivilant to select(). This allows you to perform async I/O completely from JavaScript.

pq.startReader()

This uses libuv to start a read watcher on the socket open to the backend. As soon as this socket becomes readable the pq instance will emit a readable event. It is up to you to call pq.consumeInput() one or more times to clear this read notification or it will continue to emit read events over and over and over. The exact flow is outlined [here] under the documentation for PQisBusy.

pq.stopReader()

Tells libuv to stop the read watcher on the connection socket.

pq.writable(callback:function)

Call this to make sure the socket has flushed all data to the operating system. Once the socket is writable, your callback will be called. Usefully when using PQsetNonBlocking and PQflush for async writing.

More async methods

These are all documented in detail within the libpq documentation and function almost identically.

pq.consumeInput():boolean

Reads waiting data from the socket. If the socket is not readable and you call this it will block so be careful and only call it within the readable callback for the most part.

Returns true if data was read. Returns false if there was an error. You can access error details with pq.errorMessage().

pq.isBusy():boolean

Returns true if calling pq.consumeInput() would block waiting for more data. Returns false if all data has been read from the socket. Once this returns false it is safe to call pq.getResult()

pq.setNonBlocking(nonBlocking:boolean):boolean

Toggle the socket blocking on write. Returns true if the socket's state was succesfully toggled. Returns false if there was an error.

• nonBlocking is true to set the connection to use non-blocking writes. false to use blocking writes.

pq.flush():int

Flushes buffered data to the socket. Returns 1 if socket is not write-ready at which case you should call pq.writable with a callback and wait for the socket to be writable and then call pq.flush() again. Returns 0 if all data was flushed. Returns -1 if there was an error.

listen/notify

pq.notifies():object

Checks for NOTIFY messages that have come in. If any have been received they will be in the following format:

var msg = {
  relname: 'name of channel',
  extra: 'message passed to notify command',
  be_pid: 130
}

COPY IN/OUT

pq.putCopyData(buffer:Buffer):int

After issuing a successful command like COPY table FROM stdin you can start putting buffers directly into the databse with this function.

• buffer Is a required node buffer of text data such as Buffer('column1\tcolumn2\n')

Returns 1 if sent succesfully. Returns 0 if the command would block (only if you have called pq.setNonBlocking(true)). Returns -1 if there was an error sending the command.

pq.putCopyEnd([errorMessage:string])

Signals the backed your copy procedure is complete. If you pass errorMessage it will be sent to the backend and effectively cancel the copy operation.

• errorMessage is an optional string you can pass to cancel the copy operation.

Returns 1 if sent succesfully. Returns 0 if the command would block (only if you have called pq.setNonBlocking(true)). Returns -1 if there was an error sending the command.

pq.getCopyData(async:boolean):Buffer or int

After issuing a successfuly command like COPY table TO stdout gets copy data from the connection.

Returns a node buffer if there is data available.

Returns 0 if the copy is still in progress (only if you have called pq.setNonBlocking(true)). Returns -1 if the copy is completed. Returns -2 if there was an error.

• async is a boolean. Pass false to block waiting for data from the backend. defaults to false

Misc Functions

pq.escapeLiteral(input:string):string

Exact copy of the PQescapeLiteral function within libpq. Requires an established connection but does not perform any I/O.

pq.escapeIdentifier(input:string):string

Exact copy of the PQescapeIdentifier function within libpq. Requires an established connection but does not perform any I/O.