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This is documentation for Caché & Ensemble.

For information on converting to InterSystems IRIS, see the InterSystems IRIS Adoption Guide and the InterSystems IRIS In-Place Conversion Guide, both available on the WRC Distributions page (login required).

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Communication Between Caché Processes

Communication Between Caché Processes

Interjob communication (IJC) devices are a set of special device numbers that let you transfer information between two or more Caché processes. The processes can be either jobbed processes or interactive processes.

IJC devices work in pairs. You can have up to 256 IJC device pairs. You use even-numbered devices, called receivers, to read data. You use odd-numbered devices, called transmitters, to write data. Attempts to read from a transmitter or write to a receiver result in a <NODEV> error.

You issue I/O commands to IJC devices, just as to any other device. After issuing OPEN and USE commands to the device, a process can issue:

  • READ commands to a receiver device

  • WRITE commands to a transmitter device

Only one process at a time can have a device open.

Pairs are based on relative order as mapped in the Caché Device Table, which you can view and edit using the configuration options of the Management Portal.

Each pair of devices is associated with an IJC memory buffer. When a process issues a WRITE command to any odd-numbered IJC device, Caché writes the data into the buffer for that device pair. When another process issues a READ command to the even-numbered device from that pair, Caché reads the data from the same buffer.

Written data is buffered in memory in first-in, first-out fashion. If a READ occurs while the device is empty, the process that issued it suspends until another process issues a corresponding WRITE. A WRITE that occurs while the buffer is full suspends until another process reads from that buffer.

After you write a message to the buffer, it remains there until it is read, even if you close the transmitter. Several users can issue OPEN, USE, WRITE, and CLOSE commands to a transmitter, one at a time in turn. Subsequent READ commands get all of the messages in the order in which they were written.

Specifying Memory Buffers for Interjob Communication Devices

The system manager can configure the IJC buffers using the Management Portal. Select System, Configuration, Advanced Memory Settings. The two parameters that can be set are:

  • ijcnum: The maximum number of IJC devices. The range is from 0 through 256. The default is 16. If you edit this setting, you must restart Caché to apply the change.

  • ijcbuff: The maximum size (in bytes) of each IJC buffer. The range is from 512 through 8192. The default size is 512 bytes. If you edit this setting, you must restart Caché to apply the change.

Each IJC device corresponds to one IJC buffer of the size specified in ijcbuff. You can write a message of length ijcbuff minus 1.

Disabling Interjob Communication Buffers

If you will not be using IJC devices, you can set the maximum number of IJC devices (ijcnum) to 0 to avoid tying up memory.

Interjob Communication Device Numbers

Interjob communication devices are automatically defined numbered by Caché. Their actual identification numbers depends on the maximum number of IJC buffers configured on the system.

The table below gives the ranges of IJC device numbers that are available on your system, depending on the number of IJC buffers that you have allocated.

For example, if you allocate 8 IJC buffers, then device numbers from 224 through 239 are defined on the system (even numbers for READ devices and odd numbers for WRITE devices).

As another example, if you allocate 94 IJC buffers, then the following range of device numbers are defined: 224 through 255, 64 through 199, 4 through 19, and 2048 through 2051. You can use any even/odd number pairs with OPEN, USE, READ, WRITE, and CLOSE commands.

IJC Device Numbers
Buffers Allocated READ Device # WRITE Device #
1 224 225
2 226 227
3 228 229
... ...  
15 252 253
16 254 255
17 64 65
18 66 67
... ... ...
83 196 197
84 198 199
85 4 5
86 6 7
87 8 9
88 10 11
89 12 13
90 14 15
91 16 17
92 18 19
93 2048 2049
94 2050 2051
95 2052 2053
... ... ...
254 2370 2371
255 2372 2373
256 2374 2375

I/O Commands for IJC Devices

You use all of the standard I/O commands with IJC devices: OPEN, USE, READ, WRITE, and CLOSE.

OPEN Command

The OPEN command reserves interjob communication devices for your use.

OPEN device::timeout


device A device number from the table above. OPEN an even-numbered device to issue READ commands. OPEN an odd-numbered device to issue WRITE commands. For two processes to communicate, they must open a set of device pairs.
timeout Optional — A positive integer whose value in seconds is the longest time Caché waits for an OPEN to finish. If you specify 0, the OPEN returns control to the process immediately.

This example shows how two processes communicate by opening separate devices for reading and writing:

Process A                          Process B 
OPEN 227 USE 227 WRITE "MSG_1"
WRITE "MSG_2"                       OPEN 226 USE 226 READ X
CLOSE 227                           CLOSE 226
OPEN 224 USE 224 READ X             WRITE X
CLOSE 224                           MSG_1
WRITE X                             .
MSG_3                               .
                                    OPEN 225 USE 225 WRITE "MSG_3" 
                                    CLOSE 225
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Process A begins by opening device 227 and writing MSG_1 to it. Caché writes this message into the buffer shared by devices 226 and 227. Process A then writes a second message to the same buffer. Now Process B opens companion device 226 and reads the first message (MSG_1) from the buffer.

Now Process A wants to read a message, so it must open a different device, 224. Because the buffer for this device and its companion, 225, is currently empty, Process A waits until Process B opens device 225 and writes MSG_3 to it. After Caché places this message in the buffer shared by devices 224 and 225, the READ command to device 224 succeeds.

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