New mempool TX is always detected a few ms ahead of new block

[sergey-chernikov]

We're running the latest elementsd on prod liquid chain, on Linux, using ZeroMQ notification channels to detect ZCs and new blocks, and always see the same picture:

[11/27/25 08:49:11.240] hashtx: c67a4a64fb33e68b83358a2c91748ce05d2cb0a0cb3c15ba7998d7a8841a4a89
[11/27/25 08:49:11.255] hashblock: de904b5995121e12148dbc0bc82f10925056fdd2aaf5ed0366a267e08ecc2c71
[11/27/25 09:43:11.522] hashtx: b2bac69b345f241ec38e887d08803fff564144f9ad81ad76cca66d3dc775bed3
[11/27/25 09:43:11.546] hashblock: 723b1b5a9b9f850553a8af228af16d8fd228c301a0d62bd3192a9cf5b5d23c55

while the sender sees the TX in mempool for a few dozens of seconds before that.
Is there a way to improve the ZC detection speed? Is elements using async method to detect ZCs in the chain, or just polling for new blocks?

[tomt1664]

Do you have either -blocksonly=1 or -maxmempool=0 in your config?

Try:

elements-cli getmempoolinfo
elements-cli getnetworkinfo | grep relay
elements-cli getpeerinfo

[sergey-chernikov]

our config:

chain=liquidv1
[liquidv1]
# General settings:
listen=1
txindex=1
validatepegin=0
anyonecanspendaremine=0
initialfreecoins=2100000000000000
con_dyna_deploy_start=0
con_max_block_sig_size=150
checkblockindex=0
fallbackfee=0.00000100
daemon=0
con_has_parent_chain=0
parentgenesisblockhash=NULL
pubkeyprefix=36
scriptprefix=19
blindedprefix=23
bech32_hrp=tex
blech32_hrp=tlq
pchmessagestart=410edd62
dynamic_epoch_length=1000
signblockscript=51210217e403ddb181872c32a0cd468c710040b2f53d8cac69f18dad07985ee37e9a7151ae
evbparams=dynafed:0:::

printtoconsole=0

rpcuser=bitcoin
rpcpassword=xxx
rpcport=7039
rpcconnect=127.0.0.1

We run it like this:

elementsd -datadir=/bitcoin/.elements
        -validatepegin=0 -server -rest -rpcbind=0.0.0.0:7039 -rpcuser=bitcoin -rpcpassword=${RPC_PASSWORD}
        -rpcallowip=0.0.0.0/0 -zmqpubrawtx=tcp://0.0.0.0:29000 -zmqpubrawblock=tcp://0.0.0.0:29000
        -zmqpubhashtx=tcp://0.0.0.0:29000 -zmqpubhashblock=tcp://0.0.0.0:29000 -zmqpubsequence=tcp://0.0.0.0:29000
        -conf=/root/.elements/elements.conf

# elements-cli getmempoolinfo
{
  "loaded": true,
  "size": 0,
  "bytes": 0,
  "usage": 96,
  "total_fee": 0.00000000,
  "maxmempool": 300000000,
  "mempoolminfee": 0.00000101,
  "minrelaytxfee": 0.00000101,
  "unbroadcastcount": 0
}

# elements-cli getnetworkinfo | grep relay
  "localrelay": true,
  "relayfee": 0.00000101,

[sergey-chernikov]

# elements-cli getpeerinfo
[
  {
    "id": 0,
    "addr": "35.196.16.254:7042",
    "addrbind": "172.28.1.100:40812",
    "addrlocal": "51.158.xxx.xxx:40812",
    "network": "ipv4",
    "services": "0000000000000409",
    "servicesnames": [
      "NETWORK",
      "WITNESS",
      "NETWORK_LIMITED"
    ],
    "relaytxes": false,
    "lastsend": 1764316148,
    "lastrecv": 1764316211,
    "last_transaction": 0,
    "last_block": 1764316151,
    "bytessent": 65467,
    "bytesrecv": 15105799,
    "conntime": 1764260700,
    "timeoffset": 0,
    "pingtime": 0.095902,
    "minping": 0.095717,
    "version": 70016,
    "subver": "/Elements Core:23.3.0/",
    "inbound": false,
    "bip152_hb_to": true,
    "bip152_hb_from": false,
    "startingheight": 3644176,
    "synced_headers": 3645102,
    "synced_blocks": 3645102,
    "inflight": [
    ],
    "addr_processed": 0,
    "addr_rate_limited": 0,
    "permissions": [
    ],
    "minfeefilter": 0.00000000,
    "bytessent_per_msg": {
      "getblocktxn": 25371,
      "getdata": 61,
      "getheaders": 1117,
      "headers": 8959,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 99,
      "sendheaders": 24,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "bytesrecv_per_msg": {
      "blocktxn": 13341775,
      "cmpctblock": 1729991,
      "feefilter": 32,
      "getheaders": 1117,
      "headers": 2958,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "connection_type": "block-relay-only"
  },
  {
    "id": 1,
    "addr": "103.99.168.204:7042",
    "addrbind": "172.28.1.100:36768",
    "addrlocal": "51.158.xxx.xxx:36768",
    "network": "ipv4",
    "services": "0000000000000409",
    "servicesnames": [
      "NETWORK",
      "WITNESS",
      "NETWORK_LIMITED"
    ],
    "relaytxes": false,
    "lastsend": 1764316211,
    "lastrecv": 1764316211,
    "last_transaction": 0,
    "last_block": 1764310331,
    "bytessent": 169095,
    "bytesrecv": 2292860,
    "conntime": 1764260701,
    "timeoffset": 0,
    "pingtime": 0.08531,
    "minping": 0.084027,
    "version": 70016,
    "subver": "/Elements Core:23.3.0/",
    "inbound": false,
    "bip152_hb_to": true,
    "bip152_hb_from": false,
    "startingheight": 3644176,
    "synced_headers": 3645102,
    "synced_blocks": 3645102,
    "inflight": [
    ],
    "addr_processed": 0,
    "addr_rate_limited": 0,
    "permissions": [
    ],
    "minfeefilter": 0.00000000,
    "bytessent_per_msg": {
      "getblocktxn": 836,
      "getdata": 61,
      "getheaders": 1117,
      "headers": 137122,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 99,
      "sendheaders": 24,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "bytesrecv_per_msg": {
      "blocktxn": 530788,
      "cmpctblock": 1728015,
      "feefilter": 32,
      "getheaders": 1117,
      "headers": 2982,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "connection_type": "block-relay-only"
  },
  {
    "id": 3,
    "addr": "35.237.147.21:7042",
    "addrbind": "172.28.1.100:58352",
    "addrlocal": "51.158.xxx.xxx:58352",
    "network": "ipv4",
    "services": "0000000000000409",
    "servicesnames": [
      "NETWORK",
      "WITNESS",
      "NETWORK_LIMITED"
    ],
    "relaytxes": true,
    "lastsend": 1764316202,
    "lastrecv": 1764316211,
    "last_transaction": 1764311540,
    "last_block": 1764316211,
    "bytessent": 164325,
    "bytesrecv": 13410825,
    "conntime": 1764260720,
    "timeoffset": -1,
    "pingtime": 0.096914,
    "minping": 0.09625300000000001,
    "version": 70016,
    "subver": "/Elements Core:23.3.0/",
    "inbound": false,
    "bip152_hb_to": true,
    "bip152_hb_from": false,
    "startingheight": 3644177,
    "synced_headers": 3645102,
    "synced_blocks": 3645102,
    "inflight": [
    ],
    "addr_processed": 198,
    "addr_rate_limited": 0,
    "permissions": [
    ],
    "minfeefilter": 0.00000100,
    "bytessent_per_msg": {
      "addrv2": 724,
      "feefilter": 32,
      "getaddr": 24,
      "getblocktxn": 7300,
      "getdata": 51534,
      "getheaders": 1117,
      "headers": 67071,
      "inv": 6564,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 99,
      "sendheaders": 24,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "bytesrecv_per_msg": {
      "addrv2": 5422,
      "blocktxn": 2831038,
      "cmpctblock": 874513,
      "feefilter": 32,
      "getheaders": 1117,
      "headers": 676649,
      "inv": 164324,
      "notfound": 4148,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "tx": 8823656,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "connection_type": "outbound-full-relay"
  },
  {
    "id": 6,
    "addr": "103.165.192.208:7042",
    "addrbind": "172.28.1.100:43748",
    "addrlocal": "51.158.xxx.xxx:43748",
    "network": "ipv4",
    "services": "0000000000000409",
    "servicesnames": [
      "NETWORK",
      "WITNESS",
      "NETWORK_LIMITED"
    ],
    "relaytxes": true,
    "lastsend": 1764316211,
    "lastrecv": 1764316207,
    "last_transaction": 1764315987,
    "last_block": 0,
    "bytessent": 543683,
    "bytesrecv": 5195039,
    "conntime": 1764260745,
    "timeoffset": 0,
    "pingtime": 0.006838,
    "minping": 0.00673,
    "version": 70016,
    "subver": "/Elements Core:23.2.7/",
    "inbound": false,
    "bip152_hb_to": false,
    "bip152_hb_from": false,
    "startingheight": 3644177,
    "synced_headers": 3645100,
    "synced_blocks": 3645100,
    "inflight": [
    ],
    "addr_processed": 301,
    "addr_rate_limited": 0,
    "permissions": [
    ],
    "minfeefilter": 0.00000100,
    "bytessent_per_msg": {
      "addrv2": 704,
      "feefilter": 32,
      "getaddr": 24,
      "getdata": 26406,
      "getheaders": 1117,
      "headers": 478336,
      "inv": 7138,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "bytesrecv_per_msg": {
      "addrv2": 7955,
      "feefilter": 32,
      "getheaders": 1117,
      "headers": 1058230,
      "inv": 163683,
      "notfound": 2703,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "tx": 3931393,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "connection_type": "outbound-full-relay"
  },
  {
    "id": 7,
    "addr": "220.132.135.54:7042",
    "addrbind": "172.28.1.100:39166",
    "addrlocal": "51.158.xxx.xxx:39166",
    "network": "ipv4",
    "services": "0000000000000408",
    "servicesnames": [
      "WITNESS",
      "NETWORK_LIMITED"
    ],
    "relaytxes": true,
    "lastsend": 1764316211,
    "lastrecv": 1764316211,
    "last_transaction": 1764299782,
    "last_block": 0,
    "bytessent": 1426250,
    "bytesrecv": 5290196,
    "conntime": 1764260747,
    "timeoffset": 0,
    "pingtime": 0.215425,
    "minping": 0.199251,
    "version": 70016,
    "subver": "/Elements Core:23.3.0/",
    "inbound": false,
    "bip152_hb_to": false,
    "bip152_hb_from": false,
    "startingheight": 3644177,
    "synced_headers": 3645102,
    "synced_blocks": 3645102,
    "inflight": [
    ],
    "addr_processed": 133,
    "addr_rate_limited": 0,
    "permissions": [
    ],
    "minfeefilter": 0.00000100,
    "bytessent_per_msg": {
      "addrv2": 664,
      "feefilter": 32,
      "getaddr": 24,
      "getdata": 22973,
      "getheaders": 1117,
      "headers": 1363705,
      "inv": 7809,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "bytesrecv_per_msg": {
      "addrv2": 2538,
      "feefilter": 32,
      "getheaders": 1117,
      "headers": 1344327,
      "inv": 162757,
      "notfound": 1561,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "tx": 3747938,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "connection_type": "outbound-full-relay"
  },
  {
    "id": 8,
    "addr": "35.231.10.147:7042",
    "addrbind": "172.28.1.100:48148",
    "addrlocal": "51.158.xxx.xxx:48148",
    "network": "ipv4",
    "services": "0000000000000409",
    "servicesnames": [
      "NETWORK",
      "WITNESS",
      "NETWORK_LIMITED"
    ],
    "relaytxes": true,
    "lastsend": 1764316211,
    "lastrecv": 1764316211,
    "last_transaction": 1764315725,
    "last_block": 0,
    "bytessent": 475039,
    "bytesrecv": 11379989,
    "conntime": 1764260749,
    "timeoffset": -1,
    "pingtime": 0.098289,
    "minping": 0.096093,
    "version": 70016,
    "subver": "/Elements Core:23.3.0/",
    "inbound": false,
    "bip152_hb_to": false,
    "bip152_hb_from": false,
    "startingheight": 3644177,
    "synced_headers": 3645102,
    "synced_blocks": 3645102,
    "inflight": [
    ],
    "addr_processed": 208,
    "addr_rate_limited": 0,
    "permissions": [
    ],
    "minfeefilter": 0.00000100,
    "bytessent_per_msg": {
      "addrv2": 400,
      "feefilter": 32,
      "getaddr": 24,
      "getdata": 56236,
      "getheaders": 1117,
      "headers": 381472,
      "inv": 5832,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "bytesrecv_per_msg": {
      "addrv2": 5530,
      "feefilter": 32,
      "getheaders": 1117,
      "headers": 1380093,
      "inv": 166471,
      "notfound": 4392,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "tx": 9792428,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "connection_type": "outbound-full-relay"
  },
  {
    "id": 9,
    "addr": "176.112.180.135:7042",
    "addrbind": "172.28.1.100:56746",
    "addrlocal": "51.158.xxx.xxx:56746",
    "network": "ipv4",
    "services": "0000000000000408",
    "servicesnames": [
      "WITNESS",
      "NETWORK_LIMITED"
    ],
    "relaytxes": true,
    "lastsend": 1764316211,
    "lastrecv": 1764316211,
    "last_transaction": 1764309530,
    "last_block": 0,
    "bytessent": 1183477,
    "bytesrecv": 4432840,
    "conntime": 1764260752,
    "timeoffset": 0,
    "pingtime": 0.042406,
    "minping": 0.041689,
    "version": 70016,
    "subver": "/Elements Core:23.3.1/",
    "inbound": false,
    "bip152_hb_to": false,
    "bip152_hb_from": false,
    "startingheight": 3644177,
    "synced_headers": 3645100,
    "synced_blocks": 3645100,
    "inflight": [
    ],
    "addr_processed": 119,
    "addr_rate_limited": 0,
    "permissions": [
    ],
    "minfeefilter": 0.00000100,
    "bytessent_per_msg": {
      "addrv2": 532,
      "feefilter": 32,
      "getaddr": 24,
      "getdata": 21302,
      "getheaders": 1117,
      "headers": 1122253,
      "inv": 8291,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "bytesrecv_per_msg": {
      "addrv2": 2035,
      "feefilter": 32,
      "getheaders": 1117,
      "headers": 688603,
      "inv": 160941,
      "notfound": 1439,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "tx": 3548747,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "connection_type": "outbound-full-relay"
  },
  {
    "id": 10,
    "addr": "209.146.50.202:7042",
    "addrbind": "172.28.1.100:60814",
    "addrlocal": "51.158.xxx.xxx:60814",
    "network": "ipv4",
    "services": "0000000000000409",
    "servicesnames": [
      "NETWORK",
      "WITNESS",
      "NETWORK_LIMITED"
    ],
    "relaytxes": true,
    "lastsend": 1764316215,
    "lastrecv": 1764316215,
    "last_transaction": 1764310012,
    "last_block": 0,
    "bytessent": 676555,
    "bytesrecv": 6568434,
    "conntime": 1764260754,
    "timeoffset": 0,
    "pingtime": 0.185046,
    "minping": 0.184585,
    "version": 70016,
    "subver": "/Elements Core:23.2.7/",
    "inbound": false,
    "bip152_hb_to": false,
    "bip152_hb_from": false,
    "startingheight": 3644177,
    "synced_headers": 3645102,
    "synced_blocks": 3645102,
    "inflight": [
    ],
    "addr_processed": 291,
    "addr_rate_limited": 0,
    "permissions": [
    ],
    "minfeefilter": 0.00000100,
    "bytessent_per_msg": {
      "addrv2": 200,
      "feefilter": 32,
      "getaddr": 24,
      "getdata": 29340,
      "getheaders": 1117,
      "headers": 608018,
      "inv": 7898,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "bytesrecv_per_msg": {
      "addrv2": 7788,
      "feefilter": 32,
      "getheaders": 1117,
      "headers": 1368170,
      "inv": 161358,
      "notfound": 2659,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "tx": 4997384,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "connection_type": "outbound-full-relay"
  },
  {
    "id": 11,
    "addr": "161.35.136.105:7042",
    "addrbind": "172.28.1.100:45956",
    "addrlocal": "51.158.xxx.xxx:45956",
    "network": "ipv4",
    "services": "0000000000000409",
    "servicesnames": [
      "NETWORK",
      "WITNESS",
      "NETWORK_LIMITED"
    ],
    "relaytxes": true,
    "lastsend": 1764316215,
    "lastrecv": 1764316215,
    "last_transaction": 1764311583,
    "last_block": 0,
    "bytessent": 529309,
    "bytesrecv": 1631364,
    "conntime": 1764260755,
    "timeoffset": -793,
    "pingtime": 0.07761899999999999,
    "minping": 0.07736700000000001,
    "version": 70016,
    "subver": "/Elements Core:23.99.0/",
    "inbound": false,
    "bip152_hb_to": false,
    "bip152_hb_from": false,
    "startingheight": 3644177,
    "synced_headers": 3645102,
    "synced_blocks": 3645102,
    "inflight": [
    ],
    "addr_processed": 143,
    "addr_rate_limited": 0,
    "permissions": [
    ],
    "minfeefilter": 0.00000100,
    "bytessent_per_msg": {
      "addrv2": 320,
      "feefilter": 32,
      "getaddr": 24,
      "getdata": 1497,
      "getheaders": 1117,
      "headers": 488781,
      "inv": 7612,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "bytesrecv_per_msg": {
      "addrv2": 2719,
      "feefilter": 32,
      "getheaders": 1117,
      "headers": 1380093,
      "inv": 3670,
      "ping": 14816,
      "pong": 14816,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "tx": 213806,
      "verack": 24,
      "version": 133,
      "wtxidrelay": 24
    },
    "connection_type": "outbound-full-relay"
  },
  {
    "id": 12,
    "addr": "51.158.62.115:7042",
    "addrbind": "172.28.1.100:44632",
    "addrlocal": "51.158.xxx.xxx:44632",
    "network": "ipv4",
    "services": "0000000000000409",
    "servicesnames": [
      "NETWORK",
      "WITNESS",
      "NETWORK_LIMITED"
    ],
    "relaytxes": true,
    "lastsend": 1764316211,
    "lastrecv": 1764316107,
    "last_transaction": 1764310123,
    "last_block": 0,
    "bytessent": 570830,
    "bytesrecv": 7289701,
    "conntime": 1764260890,
    "timeoffset": 0,
    "pingtime": 0.010978,
    "minping": 0.010165,
    "version": 70016,
    "subver": "/Elements Core:23.2.7/",
    "inbound": false,
    "bip152_hb_to": false,
    "bip152_hb_from": false,
    "startingheight": 3644179,
    "synced_headers": 3645100,
    "synced_blocks": 3645100,
    "inflight": [
    ],
    "addr_processed": 250,
    "addr_rate_limited": 0,
    "permissions": [
    ],
    "minfeefilter": 0.00000100,
    "bytessent_per_msg": {
      "addrv2": 656,
      "feefilter": 32,
      "getaddr": 24,
      "getdata": 33482,
      "getheaders": 1117,
      "headers": 499204,
      "inv": 6517,
      "ping": 14752,
      "pong": 14752,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "bytesrecv_per_msg": {
      "addrv2": 6365,
      "feefilter": 32,
      "getheaders": 1117,
      "headers": 1120794,
      "inv": 163951,
      "notfound": 2975,
      "ping": 14752,
      "pong": 14752,
      "sendaddrv2": 24,
      "sendcmpct": 66,
      "sendheaders": 24,
      "tx": 5964669,
      "verack": 24,
      "version": 132,
      "wtxidrelay": 24
    },
    "connection_type": "outbound-full-relay"
  }
]

[tomt1664]

I've used your exact config and receive unconfirmed/ZC transactions in the mempool as the appear, exactly in sync with https://liquid.network/mempool-block/0

You are connected to plenty of relaying peers, so that seems fine.

Your getmempoolinfo return shows an empty mempool, (which could be just after a block has been confirmed) but maybe monitor this to just make sure transactions are appearing in between block confirmations.

One thing that doesn't make sense is that the minrelaytxfee is showing as 0.00000101 whereas in your config file it is set to 0.00000100 - can you confirm this? You may be missing some unconfirmed transactions with a 0.000001 rate.

But it seems to me the problem is likely your ZeroMQ client is connecting to the wrong notification type or is maybe misinterpreting the messages?

You are using these flags (all on the same port 29000):

-zmqpubrawtx=tcp://0.0.0.0:29000
-zmqpubrawblock=tcp://0.0.0.0:29000
-zmqpubhashtx=tcp://0.0.0.0:29000
-zmqpubhashblock=tcp://0.0.0.0:29000
-zmqpubsequence=tcp://0.0.0.0:29000

All the topics are multiplexed on the same TCP port, but each topic has a different message format.

If your subscriber does something like this

Pythonsocket.subscribe("hashtx")

you will actually receive every message (rawtx, rawblock, hashtx, hashblock, sequence) and then you have to look at the first frame to see which topic it really is.

Possibly you are only looking at the second frame (the hash) and assume the topic from the order they arrive, or they mistakenly treat every message as “hashtx”/“hashblock”. When a transaction gets confirmed in a block, Elements sends both a rawtx/hashtx (for mempool entry). If your code throws away or ignores the early hashtx message, you will only notice the transaction when the block arrives.

[sergey-chernikov]

Hi Tom! Thank you for your extended reply! We're connecting to ZMQ channel using C++ code (and native ZMQ lib), and parse each topic accordingly, so I don't see where the issue could be there. The events are distinct (hashtx and hashblock) and arrive at different time, yet very close. Maybe I'll try to use separate port/channel for each topic.
The moment with fees is not clear: we don't have minrelaytxfee set in the config, only the fallback fee, which acts only when funding transactions on our side.