RecSync

The record synchronizer project includes two parts. A client (RecCaster) which runing as part of an EPICS IOC, and a server (RecCeiver) which is a stand alone daemon. Together they work to ensure the the server(s) have a complete list of all records currently provided by the client IOCs.

Information Uploaded

The RecCaster client currently sends the following information about its Process Database.

• The EPICS Base Version
• A white listed set of environment variables
• The name, type, and description of all records
• Any info() tags associated with these records
• Any additional environment variables specified in addReccasterEnvVars iocsh calls

RecCaster Usage

The RecCaster source in the client/ sub-directory is build as an EPICS support module. It provides reccaster.dbd and the reccaster library. The client demo IOC in client/demoApp/ provides an example of how to build RecCaster into an IOC.

RecCaster requires only EPICS Base 3.15 and later 3.14 releases.

RecCeiver Usage

The RecCeiver server in the server/ directory is a Python script using the Twisted networking library. It requires Python 3.6 or above and Twisted >= 12.0.

The server uses the Twisted plugin interface to make client information available to one or more plugins. See server/demo.conf for an example configuration.

Currently two plugins are provided: show which print client information to screen/log, and db which writes into a SQL database (currently only sqlite3 supported). The SQL table schema used is defined in server/recceiver.sqlite3.

Theory of Operation

RecCaster

The RecCaster client is an epics support module which automatically starts a single thread and begins passively waiting for a RecCeiver server to announce its presence.

Upon receiving an announcement the client will connect and upload its list of records and related information to the server. It then remains connected indefinately and responds to periodic alive tests from the server.

If this connection is lost the client will resume waiting for another announcement.

RecCeiver

The RecCeiver server periodially (every 15 seconds) sends a broadcast announcement of it existance.

When a client connects it will wait for a server handshake message before beginning to upload records. The server will delay sending this message if too many clients are currently uploading.

UDP Protocol

Server announcement are made to UDP port 5049.

When sent to a IPv4 address, such messages will have a length >= 16 bytes. Clients will process only the first 16 bytes and ignore any remaining bytes.

0	1	2	3	4	5	6	7	8	9	A	B	C	D	E	F
ID		0	X	SERV ADDR				PORT		X	X	SERV KEY

• All multibyte fields have big-endian ordering.
• Fields marked 0 must be zero and clients will (silently) reject messages where these fields are not zero.
• Fields marked X should be ignored by clients.
• The ID field must have the value 0x5243 (ascii 'RC').
• The SERV ADDR and PORT fields is the IPv4 address and TCP port to which clients should connect.
• SERV KEY is an arbitrary number which the client will later on the TCP connection.

TCP Protocol

The TCP data stream will consist of a sequence of messages having an 8 byte header followed by a variable length body. The header will have the form

0	1	2	3	4	5	6	7
ID		MSGID		LEN

• All multibyte fields have big-endian ordering.
• The ID field must have the value 0x5243 (ascii 'RC').
• The MSGID field identifies the type of message to follow.
• The LEN field gives the length in bytes or by message body to follow the header.
• Messages from server to client will have MSGID >= 0x8000.
• Messages from client to server will have MSGID < 0x8000.
• Except where explicitly stated, endpoints must silently ignore messages with unknown MSGID
• Endpoints must be prepared to receive and ignore extra body bytes.

Messages

MSGID	Min. LEN	Name	0	1	2	3	4	5	6	7
0x8001	1	Server Greet	0
0x8002	4	Ping	NONCE
0x0001	8	Client Greet	0	0	X	X	SERV KEY
0x0002	4	Pong	NONCE
0x0003	9	Add Record	RECID				ATYPE	RTLEN	RNLEN		RTYPE	RNAME
0x0004	4	Del Record	RECID
0x0005	4	Upload Done	0	0	0	0
0x0006	9	Add Info	RECID				KEYLEN	X	VALEN		KEY	VALUE

• NONCE is an number choosen by the server for a Ping message. It must be echoed back by the client in a Pong message.
• SERV KEY must be the number the client received in the announcement from which it found this server.
• RECID is an identifier choosen by the client to identify a single record instance. Except in Add Info, must be >0.
• ATYPE is 0 to add a record entry or 1 to add a record alias. When adding an alias the record type is omitted (RTLEN==0) and the RECID must have been added in a previous message with ATYPE==0.
• RTLEN, RNLEN, KEYLEN, and VALEN are lengths in bytes.
• RTLEN and VALEN are allowed to be zero.
• RNLEN and KEYLEN must be greater than zero.
• RTYPE, RNAME, KEY, and VALUE are variable length fields whos length is given by the corresponding *LEN field.
• RTYPE, RNAME, KEY, and VALUE should not include a 0 (ascii null) byte.
• The RECID field of the Add Info may be zero. When zero the key/value pair is considered to be associated with the client as a whole, and not any individual record.

When a TCP connection is established the client must send the Client Greeting message immediately, then wait to receive the Server Greeting message before sending any further messages.

Once the client has recevieved the Server Greeting it may send begin sending Add Record, Add Info, and Del Record messages. Once the initial upload has been completed, the client must send Upload Done.

After Upload Done is received the server will periodically send Ping messages. The client must respond to each Ping with a Pong. If a client does not respond promptly the server may close the connection.