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Streams: specify better how the master enty works.

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This commit is contained in:
antirez 2017-09-27 23:12:43 +02:00
parent 7d0d9693c1
commit cea421a021
1 changed files with 25 additions and 1 deletions
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26
src/t_stream.c
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@ -177,7 +177,29 @@ int streamAppendItem(stream *s, robj **argv, int numfields, streamID *added_id,
uint64_t rax_key[2]; /* Key in the radix tree containing the listpack.*/
streamID master_id; /* ID of the master entry in the listpack. */
/* Create a new listpack and radix tree node if needed. */
/* Create a new listpack and radix tree node if needed. Note that when
* a new listpack is created, we populate it with a "master entry". This
* is just an ID and a set of fields that is taken as refernce in order
* to compress the stream entries that we'll add inside the listpack.
*
* Note that while we use the first added entry ID and fields to create
* the master entry, the first added entry is NOT represented in the master
* entry, which is a stand alone object. But of course, the first entry
* will compress well because it's used as reference.
*
* The master entry is composed of just: an ID and a set of fields, like:
*
* +------------+------------+---------+---------+--/--+---------+
* | 128 bit ID | num-fields | field_1 | field_2 | ... | field_N |
* +------------+------------+---------+---------+--/--+---------+
*
* The real entries will be encoded with an ID that is just the
* millisecond and sequence difference compared to the master entry
* (delta encoding), and if the fields of the entry are the same as
* the master enty fields, the entry flags will specify this fact
* and the entry fields and number of fields will be omitted (see later
* in the code of this function). */
int flags = STREAM_ITEM_FLAG_NONE;
if (lp == NULL || lp_bytes > STREAM_BYTES_PER_LISTPACK) {
master_id = id;
@ -221,6 +243,8 @@ int streamAppendItem(stream *s, robj **argv, int numfields, streamID *added_id,
memcmp(e,field,e_len) != 0) break;
lp_ele = lpNext(lp,lp_ele);
}
/* All fields are the same! We can compress the field names
* setting a single bit in the flags. */
if (i == master_fields_count) flags |= STREAM_ITEM_FLAG_SAMEFIELDS;
}
}